Remember that harder questions count for more points. So, if you got a lot of questions correct and the beginning of your test, you got yourself up into the harder material. It's definitely plausible that you could get half of the questions wrong towards the end of the test and still score really well.

The computer adaptive wrinkle on the GMAT makes analyzing your percent right/wrong very difficult - that's why practice tests are so important. GMAT Prep tends to be very accurate, so it sounds like you are in good shape.