When Justice Samuel Alito was nominated to the U.S. Supreme Court by President George W. Bush 10 years ago, it represented a triumph for the conservative legal movement. Haunted for decades by the ghosts of Supreme Court nominees they found disappointing—most notably, David Souter and Anthony Kennedy—conservatives finally had their man. As the law professor Jonathan Turley noted when Alito was nominated, “No one on the conservative base can be unhappy with Sam Alito.” A decade into his tenure on the Court, Alito hasn’t disappointed.

Alito is often compared to the justice he succeeded, Sandra Day O’Connor. Sitting at the Court’s center, O’Connor held the critical vote on a range of important issues, from affirmative action to reproductive choice to campaign finance. To varying degrees, Alito has voted to reverse course in each of these areas. And with blockbuster cases on affirmative action and reproductive choice on the Court’s docket this term, he will have more opportunities to leave his ideological mark.

But although the familiar comparison to O’Connor demonstrates Alito’s conservatism, a comparison to Chief Justice John Roberts helps explain why Alito’s record is the most consistently conservative of any justice on the current Court. Superficially, the two men are quite similar. Both joined the Court when they were in their 50s. Both had sterling academic credentials. Both spent their early professional careers in the Reagan Administration. But even as both have had very conservative records while on the Court, Alito’s has been meaningfully more conservative, particularly in the last few years.

The reason why may lie in one big similarity and one big difference between the two justices. First, the similarity: When it comes to interpreting the law, neither justice has a set methodology or approach. Roberts was quite candid about this during his two confirmation hearings, explaining at his Supreme Court hearing that he does “not have an all-encompassing approach to constitutional interpretation” and that he “would follow the approach or approaches that seem most suited in the particular case.” In a recent interview with the American Spectator, Alito made a similar comment, noting, “I start out with originalism ... But when you have to apply that to things like a GPS that nobody could have dreamed of then, I think all you have is the principle and you have to use your judgment to apply it.”

This similarity leaves ample room in individual cases for the votes of these justices to be shaped not by the law, but by their ideological convictions. Indeed, for both justices, there are numerous areas in which their votes are difficult to reconcile with what the law apparently required. In Shelby County v. Holder, for example, Roberts wrote an opinion (in which Alito joined) invalidating a key provision of the Voting Rights Act; to reach that result, Roberts all but ignored the very constitutional provision that authorized Congress to enact the Voting Rights Act in the first place. In Gonzales v. Carhart, both justices joined an opinion upholding a federal law restricting abortions that was materially identical to a state law the Court had struck down just seven years earlier. In her oral dissent from the bench, Justice Ginsburg excoriated the majority for ignoring the Court’s precedent and pointed to the change in the Court’s composition since its previous decision (notably, the addition of Roberts and Alito to the bench) as key to its decision.

And Alito hasn’t just been a silent partner in the Roberts Court’s conservative coalition. In many key cases, he has been its confident leader. Alito, for example, wrote the Court’s 5-4 decision in Ledbetter v. Goodyear Tire & Rubber Co., which held that a woman could not pursue a claim of pay discrimination against her employer because, under the majority’s reading of the law, she took too long to bring it. As Ginsburg wrote in dissent, the contrary conclusion, recognizing that sometimes employees are not immediately aware of wage inequities, would have been “more faithful to precedent, more in tune with the realities of the workplace, and more respectful of [the governing law’s] remedial purpose.” Congress later passed a law endorsing Ginsburg’s take and overriding the Court’s decision.

Alito also wrote the Court’s 5-4 decision in another significant case affecting women’s rights. In Burwell v. Hobby Lobby Stores, the Court allowed for-profit companies to opt out of providing contraceptive coverage to their employees, as required by the Affordable Care Act, if the companies’ owners objected to the use of contraceptives for religious reasons. It was a hugely significant decision, both for its extension of free exercise rights to for-profit corporations and for its conclusion that those rights can trump the rights of employees to enjoy benefits provided by federal law. (Significantly, the Court is deciding another case involving the ACA’s contraceptive coverage mandate this term.)

Finally, Alito authored two majority opinions that questioned a nearly 40-year-old Supreme Court precedent that allows public-sector unions to collect a fair share of the cost of collective bargaining from all government employees who benefit from that bargaining, even if they’re not union members. Alito’s push to overturn this landmark decision represents a major threat to public-employee unions nationwide. And no doubt thanks to Alito’s encouragement, the Court is currently considering whether to overrule that 40-year-old precedent in Friedrichs v. California Teachers Association.

In the end, both Roberts and Alito are solidly conservative justices—willing to use a range of interpretative tools to push the law to the right in a number of critical areas. But there is one significant difference between the two justices, and that difference helps explain why their records have been materially different in their first decade on the Court. As we concluded after taking a year-long look at Roberts’s first decade as chief justice, Roberts is deeply concerned about the institutional legitimacy of the Court—since joining the Court, he’s repeatedly expressed the concern that the Court not be seen as an extension of the political process—and there are areas in which that concern trumps his ideology. In contrast, Alito’s record is almost always consistent with his conservative ideology.

Perhaps the most prominent examples of this Roberts-Alito split are the two health-care reform cases in which Roberts, but not Alito, voted to uphold, at least in substantial part, the Affordable Care Act. In the first case, Roberts cast the deciding vote to uphold the ACA’s individual mandate against constitutional challenge and, in the second, he joined Justice Kennedy and the Court’s more progressive members to hold that the tax subsidies that put the “affordable” into the Affordable Care Act should be available nationwide. Alito dissented in both cases.

Roberts and Alito were also on opposite sides in United States v. Arizona, another case in which the institutional stakes for the Court were high. In that case, Roberts joined Kennedy’s opinion striking down key parts of Arizona’s anti-immigrant law, protecting the executive branch’s broad discretion when it comes to enforcing the nation’s immigration laws. (This decision is of particular salience now given the Court’s decision to hear the challenge to President Obama’s executive action on immigration this Term.)

By our count, there have been at least 10 significant cases in which Roberts parted ways with at least some of his conservative colleagues (including Alito), which is why, as one of us wrote last year, “in many important cases, progressives shouldn’t count him out.” The same can’t be said for Alito. This is not to say that Alito never votes for a result that seems to be at odds with his ideological views—he has on occasion (for example, he voted with the rest of the Court to hold that warrantless cell-phone searches following an arrest are unconstitutional, though he wrote separately and by himself to suggest that state legislatures could choose to allow such searches)—but such instances are exceedingly rare.

Even Justices Scalia and Thomas have areas (most notably, some areas of criminal procedure) in which their votes are clearly in play. But it’s difficult to say what those areas or cases might be when it comes to Alito. And that’s surely why, 10 years into his tenure, it appears there’s no one to the right of Alito on the current Court.