Hi,
The GMAT is a Computer Adaptive Test. This means that every time you answer a question correctly, it gives you a more difficult question, and every time you answer a question incorrectly, it gives you an easier question. The result is that no matter where you are in terms of your ability level, you will always be answering about half of the questions wrong. The key is to be able to answer high-level questions wrong, so that even if you're getting 800-level questions wrong, you would still be getting 700-level questions right.
You can find a detailed explanation of how the GMAT finds your score here:
http://www.manhattangmat.com/strategy-s ... -score.cfmBest,
Caitlin Clay
Student Services Associate
Manhattan GMAT