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Only you can find out though I will say don worry too much about that right now. Each of them follow close but different algorithms to determine test scores.

Probably, the questions you saw on the three tests were from different areas you are strong in.
Another reason could be that you are expecting perfect results on GMATPrep because you know inside that it mocks the tests more than other prep companies.
(I am doing critical reasoning here )

Review your tests to see why you got those scores. Check each wrong answer and weak area. You will probably get an idea why yout got those results. But important thing is to learn from your mistakes.

All d best.

The verbal scores look quite consistent, but you scored substantially worse in the quant section of GMATPrep than in the other exams. Did you make a lot of silly mistakes in your GMATPrep or run out of time? How many data sufficiency mistakes did you make?

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It's wise to make an educated guess when you feel that a question is too difficult, but take your time to read questions carefully and note what they are asking for! GMATPrep questions are worded in a way that tests your ability to do so reliably