One word: No. When people say that AP exams are marked on the curve, they don't mean an actual bell curve. What they mean by "curve" is the amount of points necessary for a certain score. This varies with the test, but has absolutely nothing
to do with an actual bell curve. For example, if a specific AP psychology exam is slightly more difficult than the others, the psychometricians (people that make the tests) will know before the test is even sent to schools. They will thus lower the curve, which means that less points are necessary for a 5.
Now, the real reason for the small amount of students with 5s in those particular exams is pretty simple. They are known for being easy, which encourages many inexperienced freshmen to take it. Also, many people tend to take AP exams just for the sake of taking them, even if they don't care about the subject at all. Obviously, their scores are less than great. Similarly, only extremely motivated and gifted students take the extremely hard exams, like Physics C or Calc. BC. These students will obviously start studying well before the exam, and will earn the score they deserve.
To put it simply, there's a higher concentration of gifted and motivated students in classes like Physics C than there are in classes like Psychology.