This is what economists call the "sticky floor" theory of unequal pay. For entry level jobs, low-paying work, and occupations for young people, there is often very little difference between male and female pay. But as men and women grow older and richer, equality gives way to inequality, and men race ahead while women's wages stick to the floor.
What explanations can I offer? One is institutional sexism; and although it's difficult to prove sexism with just a chart, it hard to argue that it's not a contributing factor. Two is what we've come to call "the mommy track." Many women take more time out of the labor force than men when they have children, and this puts them at a disadvantage for executive promotions.
Three is the shifting expectations of work and life among all married couples. A Harvard Business School study found that older women often allow their husbands' careers to take precedence later in life, even when those same women expect to nourish their own careers when they're younger. This is true, the authors said, even when those women don't have children. Whether this is a bad thing for each and every relationship is not at all for me to say. Sometimes, an ambitious female twentysomething who graduates from, say, the University of Chicago Booth School of Business grows into a happy fortysomething mom who works part-time to raise two kids while the husband's career advances. (Sometimes, in that sentence, specifically means in the case of my mom and dad.) But as a matter of pay equality, it's plausible that the shifting expectations of married life and children nudges many middle-aged women out of their career's express lane, leading to unequal pay in the C-suite that didn't exist in the proverbial (or literal) mail room.
The gender-pay gap is not a number. It is many numbers—many more than appear in this article—which are easily conflated tell a simplified story of pay equality. If you are focused on one number in the gender-pay story, you are seeing a one-dimensional representation of a multi-dimensional issue.