Those tests are different. The Official GMAT questions are the best at subtle questions with answer choices that draw you in and make you feel sure that you have answered correctly, when you have actually fallen into the trap.

The official questions benefit from two things that allow them to be so much more realistic. They are offered as experimental items first. So that GMAC knows the difficulty of the question based on actual student responses, as opposed to experts' opinions of how difficult a question is. Second, these questions are edited to a very high degree; each word is carefully selected to be tricky, yet fair.

This allows official questions to be much more subtle. You not only have to be able to do the math but you have to recognize what the problem requires.

Perhaps you are having trouble with the subtlety of the official questions?