The current Slate Star Codex, my favorite blog, has a link to an account of an interesting piece of research on student evaluations of teachers. It judged the quality of teachers by how well their students did in later courses, compared the result to student evaluations of teaching quality, and found that the two anti-correlated. On average, good teachers get bad ratings, bad teachers get good ratings. For details, follow the link.
It's a single study, I have not read the original paper, and the result might be wrong. But it is interesting to think about reasons why it might well be right.
The most obvious one is that many students don't like to work hard. A professor who does not assign much homework or reading and grades easily might get better ratings, from many although not all students, than one with the opposite pattern. My daughter, as a student at Oberlin, was struck by the fact that most of the other students in a class were happy when, for some reason, it didn't meet. The same pattern—study seen as a cost, not a benefit—might well apply here.
There is a second and less obvious possible reason. Correct ideas are frequently hard. Easy ideas are frequently wrong. My standard example is from popular discussions of foreign trade issues. Most of them take for granted a view of the economics of trade, the view implicit in terms such as "unfavorable balance of trade," that economists refer to as the theory of absolute advantage. That particular view of the subject has been obsolete for about two hundred years. But while the theory of absolute advantage does not make sense if you think about it carefully, it is considerably easier to understand than the theory of comparative advantage, which does. That is why the former was worked out first and why it has had such a successful postmortem career.
A professor who insists on telling the truth, on explaining hard ideas correctly, may well come across as a worse teacher than one who fudges, offers a simplified and less correct version. Half the students of the former end up believing that they do not entirely understand the subject being taught—and they are right. Almost all the students of the latter end up sure they understand it—and wrong.