Not everyone tests well.
I've been an educator for a long time, and one of the things that you figure out pretty quickly in my line of work is that everyone has certain strengths and weaknesses. The kids I've always felt particularly bad for are the ones that try really really really hard in class, turn in all of their homework, and study their asses off, but for whatever reason on test day just end up drawing a complete blank and bombing their quiz about robber barons or Adlai Stevenson or whatever.
It has nothing to do with their intelligence or ability, it's just that during crunch time they freeze up and.