The very first Kaplan test seems to be slightly off, or at least that's what most people report. I myself got only a 49 in their quant section, even if I only had two mistakes (two mistakes will usually take you to a 50 or 51). I say trust GMATprep for the moment and put the Kaplan results into context: how many did you get wrong? Any timing issues? Is there a certain topic that you're consistently missing in the timed conditions of the test? Analyzing the results this way will help you gain more relevant info on what you need to work on that a score.