While I can't speak to the accuracy of the Veritas
or Princeton sim tests, if you need more practice I encourage you to try a free trial of Economist
GMAT Tutor which includes one adaptive sim test. Check us out here: http://gmat.economist.com
Remember, a score can dip from one exam to another even with the same amount of knowledge ot the same percentage of correct/incorrect. The GMAT scoring algorithm takes many factors into account including where you make errors (eg making mistakes on 'graded' versus experimental questions, or the 'weight' of the question itself etc), 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 (ie did you spend only a few seconds on the last few questions as opposed to the full time you should have), how you felt on test day - these can all affect score.
It is possible that one exam may be off just because of where you made your mistakes rather than how many. But 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.
Economist GMAT Tutor