I routinely bomb the simplest interview quizzes. Usually the early "weed-out" ones like FizzBuzz. I do fine on the much more advanced questions.
I think part of what makes them more difficult for me is that they feel like a test. The more advanced questions, they feel like engineering problems in need of a solution. But when someone pops up with an out-of-nowhere "do this purpose-less thing!" question, I spend all of my time wondering about what they're actually looking for, and none thinking about the problem at hand. Is this a trick? Am I missing something? What does he want from me!?
I also just can't code worth a damn when someone is staring at me. I love interviewers who pose a long, complicated question and then leave the room for 30 minutes so I can work on it. These are always much more difficult problems, and I find them infinitely easier than the little 2-minute quizzes.