First, here's a very similar discussion on this:
gmatprep-harder-than-gmat-64513.htmlI have read that the GMATPrep is harder and that the real GMAT is harder. I have also read that the verbal on the actual test is harder, while the quant is easier. Others believe that the real test's quant is trickier. To be honest, I don't know what to think anymore!
But I wouldn't be surprised if the real GMAT is harder than the practice tests. The GMAT itself is constantly undergoing adaptation and change in response to test takers' ability to master the test structure. Question makers explore new and different ways to present problems in order to throw us off and not become too comfortable. If the test makers didn't adapt to
our ability to adapt, the GMAT would be very, very easy. GMATPrep, on the other hand, does not share such dynamics. It merely has a bank of retired test questions. It's also important to remember that the GMATPrep generally inflates one's score after the second practice test. Obviously, this can skew our expectations for the real test.
There are other factors to consider too. Nervousness and mental exhaustion will be felt more acutely at the test center; there's a lot more at stake on the real GMAT than on the practice tests, and you will definitely feel it on test day. You also take the GMAT in unfamiliar surroundings.