I balked at the idea that "the main determinant of doing well in AP classes is IQ," which I'm not so sure about, especially when conflated with the idea that IQ is also a measure of student intelligence. In my experience, AP classes are focused on test preparation, which I would argue is not so strictly bound to intelligence at all.

Probably what we're seeing is an expression of the widespread American notion that every child is special and smarter than the other kids and is bound for greatness, and that success is to be achieved through excellence in traditional educational and career channels.

Aspiration is one thing, but many parents are actually trying to force their children through the educational system to "become the next president" in some sense. We simply have a deeper well of more diverse test takers in the AP exams (i.e. more variety in preparedness), thus the trending in scores.