Some general observations about scoring discrepancies, probably not very pertinent to you but helpful to some other test takers.
MGMAT's (and that of all the other prep companies) scoring algorithm can be VERY off sometimes, depending on factors such as MGMAT's own interpretation of question difficulty, sequence of errors in a row, or the question number at the which the errors are made.
Since PearsonVUE pretty much don't reveal the exact rubrics of their scoring algorithm or CAT systems, the discrepancies between practice scores (be it MGMAT or some other prep company, or even the GMATPrep) and the actual test are not actually very surprising. These prep companies are doing their best to guess your testing level to the best of their abilities, probably with a bigger standard deviation than they might be willing to accept.
In reality, score discrepancies are often indicative that the test takers was not at the scoring level that the practice tests were suggesting and that some fundamental GMAT concepts are missing from the preparation. Bottom line is, unless the test taker was sick or nervous or had some psychological issues or even timing issues, the score that he/she receives on the real test is a reflection of his/her GMAT level. Of course, luck (guessing well on that day or getting fewer questions from your weak areas and more from your fortes) can play a part on test day and can be the difference between a 680 and a 700, or a 750 and a 770, but if you are scoring in the 740s on practice tests and the get 650 on the real test, chances are that you are not yet a 700-level and with good practice you can surely get over that hump, given the potential that you have shown on your practice tests.
Der alte Fritz.
+1 Kudos me - I'm half Irish, half Prussian.