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.