I don't think percentages in this sort of context are a good metric. People under time pressure or stress make a lot of mistakes often.
Still, doesn't 63% for calculus seem low? I think that if you know all of the material like the back of your hand, you can get 85+% right even with mistakes.
At the highschool level most kids have little idea what they enjoy or are genuinely talented at. Having them get a few tastes of more advanced material in a variety of subjects is therefore good.
A possible concern is that what people learn might be too superficial for them to get even a "taste."
Also, at a practical level, actually getting kids at that point to have an in depth understanding of subjects is often difficult. For example in the calculus case, sequences and series are one of the last things taught, and they are substantially more abstract and are simply easier to teach when students have had more hands on experience with what calculus can do.
The case of sequences and series is interesting. My impression is that most students who get 5's on AP Calculus BC really don't understand the topic. If I were designing the curriculum, I think that I would pare down that portion of the course so that students got a more gentle introduction.
85% of what? I guarantee you that I can write a calculus exam, using only AP-level material, that is long enough or algebraically messy enough that you can't get 85% of it right in three hours (or whatever the actual time limit is). There is no fundamental reason the AP exams should have exactly the same difficulty curve (5% of students can get 100%, another 10% can get over 90%, another 30% can get over 80% or whatever) as a high school class.
When I was in high school, I noticed is that it was possible to score the top mark on an Advanced Placement (AP) Exam by answering a relatively small portion of the questions correctly.
During my junior year, I self-studied calculus, and took the AP Calculus AB exam. I was very surprised that I scored a 5 (the top mark), because at the time when I took the exam, I didn't know some very basic things that were on the syllabus.
The College Board gives the raw score to AP score conversions for the exams that have been most recently released. The percentages needed to get a 5 are as follows: