GMATPrep is made by the same people who make the real test, so you should consider that test the best predictor of your score.
So what might be causing this? Have you been taking the practice tests under 100% official conditions, INCLUDING essay and IR? If you skip the essay or IR, or give yourself longer breaks sometimes, or do anything else that's not how the real test will work, then you could be artificially inflating your score on the quant and verbal.
Are you having timing issues? Timing issues can cause serious fluctuations in scores - sometimes better, sometimes worse. You can use this to analyze your MGMAT tests to see whether there are timing issues:
http://www.manhattangmat.com/blog/index ... ice-tests/What time of day did you take the practice tests? What had you been doing earlier that day and in the couple of days beforehand? Were you possibly tiring yourself out much more before those GMATPrep tests? If so, perhaps those tests are artificially lower than they should be.
Are you burning yourself out? Are you studying a ton, non-stop, and finding yourself getting sick of everything? If that's the case, you may be suffering from serious mental fatigue and you may need to give yourself a break to get your mental energy back.
What do you think?