This finding comports with my own experience, both as a student and as a professor. Even attending a state school in the Deep South, my political science and history professors were predominantly (but not exclusively) liberal. But debating them tended to reinforce my conservative leanings. Years later, teaching political science courses to predominantly conservative students, I oftentimes found myself taking a Devil’s Advocate stance simply to force them to challenge their own preconceptions. (Which, on reflection, made me wonder if my own profs hadn’t done the same thing.)
Another thing to keep in mind is that politics simply is a non-factor in most college courses. Even now, when I imagine campus politics, like that in the country as a whole, is more polarized than at any time since the Vietnam era, there’s likely not much political talk in the math, science, engineering, and foreign languages courses.
One thing to think about is that the clear biases of my English professors brewed a certain cynicism in many students. It was so obviously easy to manipulate our professors by turning in sub-standard papers that catered to their political concerns that it was hard to see those beliefs as the product of rigorous analytical thinking.