A 760 is indeed a great score, but did you practice under realistic test conditions with regards to time limits, breaks, and AWA? If not, then the score becomes much less reliable.
You are correct that the GMATPrep is the most accurate indicator of one's actual performance on the real test. But how did you perform on the Manhattan CATs? Could you provide us with the those scores and dates taken?
I would also like to commend your overall attitude regarding the GMATPrep score. Even if one scores very well on the practice test under appropriate conditions, it's crucial to not become complacent. A good score should energize and motivate you, but stay focused!
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