A score can dip from one exam to another even with the same amount of knowledge or the same percentage of correct/incorrect. The GMAT scoring algorithm takes many factors into account including where you make errors, how many mistakes you commit in a row, whether you finish both sections or leave questions out, whether the last few questions of either section seemed like they were guessed at, how you felt on test day, etc. These factors can all affect your score.
From one test to the next you can easily jump +/- 30 points (and even up to 80 or so) depending on where the mistakes are. *Where* you make mistakes can often be more important than how many. Overall, when a student wants to know where he/she stands, the best way to do so is to take the average after a few sim tests. This should get you to a fairly close estimation of where you are at.
In general, our students tend to score within about +/- 30 points or so from what they get on our tests and the GMAT test preps.
Hope this helps!
For free GMAT practice questions, follow us @econgmat_qs
Economist GMAT Tutor