"Performing at the 90% level" doesn't mean anything unless you assume that the questions are precisely calibrated so that someone with a good understanding of the material will get about 90% of the questions right in the allowed amount of time. While this is very common in high school, in my five years of college I never took a class that used this scale. Why should you assign problems so easy that someone with almost no grasp of the material gets 60% of them right (D) and a decently smart person who studies a lot can be assured of 100% (A+)?

The question you should be asking is "Is it possible to get the percentage needed for a 5 without a good understanding of the subject?" An exam with five truly difficult problems, where you get a 5 if even one is fully solved, would be a good exam by this standard despite looking very bad by the standard you are using.

I agree that AP exams are often too easy and do not require a good understanding of the material, but

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.

is the only thing in your whole post that's really relevant to this.