GMATPrep uses the same algorithm as the official test and its database of questions consists entirely of questions that have appeared on the official exam in the past.

Our perceptions of one test being harder than another are just that - perceptions. There are many reasons why someone would think one test (whether GMATPrep or official) is harder than another - the difference in prep because the tests are taken at different times, the random mix of questions you happen to get on each test, the elevated stress level of the real test vs. a practice test, etc. The tests themselves are basically the same - the only substantive difference is that you know, psychologically, that the real thing counts and the practice test doesn't.