Well, I think one of the biggest factors that contributes to people missing questions is the time issue. With only the 10-15 minutes they give you to answer 50 questions, it's not like people will immediately know the answer to every question. Some questions, because they don't have time to rack their brain, may be skipped or passed on. Or, in the haste of the test, people may just put down a wrong answer.Those questions are like sixth or eighth grade level questions. It is an indictment of our education system that a kid can go through two or three years of college and not get 100% on this test every time.
That said, I took a fake test like these once (with 50 questions) and got 47. Those guys who score below the 20's just mind boggle me.