I don't remember exactly how many AP tests I took, or what I got on them- the number 13 seems to be available, and I remember getting mostly 4s and 5s. I do remember that I got a year and a half's worth of credits out of it (which is the other benefit of taking lots of different tests), and so had almost all of my 'core credits' out of the way when I stepped onto campus.

Conspicuously, these percentages are all well below the standard 90% needed for the top mark (A) in a high school or college course.

The 90% is generally after adjustment, and with taking into account lots of "effort" points which are generally viewed by professors and students as mostly free. Many of my hard science courses in the course of my undergraduate degree had average marks on exams in the 60% range, with the grades curved such that about 30% of the class would get an A, 40% a B, and so on. (I'm sure the percentages vary widely across classes and schools.)

In that view, a better question to ask is sometimes "what percentage of students get a 5 on the AP X test?". If you look at Calculus (in 2008), the scores were split roughly uniformly on the AB test, such that about 20% of students got a 5. On BC, about 40% of students got a 5 (which I would explain by students being much more selective in deciding to take BC).