This is one very common question I have come across from many of GMAT aspirants. Having trained many students for GMAT, I always advise my students to use official tests provided by mba.com, to know accurately where they stand and the level of their preparedness for the final test, of course, they are from the makers of the test directly.
GMATPrep tests provides the most accurate estimation of your real score with a margin of error of almost +/-30 points that is someone who scored 670 on GmatPrep may well get a 700 on test, or may also end up with a score of 640, as your score on any given day also depends on your mood, sleep quality, the questions on the test, or whether you have taken the mocks in the real test-taking environment, yeah! all these matter.
Apart from official tests if you are using other tests such as Kaplan, Manhattan, Veritas, Economist the difference in the scores can be even 80-100 points, I have observed this with many of my students, its due to the fact that apart from a few features the test-maker has shared, the adaptive algorithm of test is a closely-held secret. For sure, other adaptive tests can be used to help in improving accuracy, pacing throughout the test, guessing strategy to be used that’s not getting stuck on the question instead, making a guess and move on, and very important, how to handle the test stress still being calm throughout.
Having said all, the student should put his sincere efforts on solid preparation to ace the GMAT test!