Percentages of 5s on a give test shouldn’t be viewed in a vacuum. You also need to look at how many kids take each of the tests and what types of students take them. Will tend to be taken by the best math students. You are advanced in math even to get to the class. And its beyond what many schools require. Per the AP data for 2017 (linked above though you need to click the 2017 tab), 133,000 kids took the BC exam. 317,000 kids took the AB exam. 509,000 kids took the US History exam. 55,000 kids to the Physics C mechanics exam.