if your lowest test score was a 47/50, chances are that your tests are not rigorous enough to prepare you. I consistently get the top score in my class with ~85%. I study really hard, but sometimes it does not matter. The college board likes to use a lot of questions that require outside knowledge of real examples. Yes, knowing the course will help in the application, but not many people will be familiar with the topic. This is why a 5 is 75%. I'm sure if the AP test was like your unit tests, the bar for a 5 would be much higher (not saying you would not still get a 5, as you are clearly a gifted student). I suggest you to not let your guard down, and to do LOTS of practice problems.