Here are some analysis of AP biology

http://blog.prepscholar.com/is-ap-biology-hard

http://blog.prepscholar.com/are-ap-tests-and-classes-hard

According to College Board, the percentage of testers who got 5 out of AP Biology (6.4%) is less than either AP Chemistry (9.2%) or AP Physics C (about 30%):

https://secure-media.collegeboard.org/digitalServices/pdf/research/2015/Student-Score-Distributions-2015.pdf

True, the results were particially a self-selective process since only 20-50K students took AP Physics C; while about 223K took AP biology and 154K took AP chemistry. Whoever decided to take AP Physics C was a better student on average than those who took AP biology. However, for AP US History and AP World History, which had more testers than AP biology, the perfect scorers are more likely (about 6.6% for World and 9.4% for US). 

One difference between Biology and Physics: the former requires a lot of memorization and many more sub-subjects. The latter requires more fundamental understanding of a few physics laws and better logics. I assume those 20-50K students who took AP Physics C were mostly Asian students who are good at abstract thinking.

On a side note: Is Darwin's Theory Of Evolution proven? How do genes code for behaviors? There are no concensus answers yet. If the teacher does not understand the biology theories involved, how can they teach or explain the subject well to students?