<p>The distribution of scores on AP tests likely has a lot to do with the self-selected test takers. Calculus BC has a high percentage of 5 scores because it attracts the top students in math. Meanwhile, some of the APs that are widely seen as easier ones have lower score distributions, perhaps because the not as good students choose them over those which are widely seen as harder ones, or because some high schools routinely offer them to non-prodigy freshmen and sophomores.</p>