It's a little odd but not necessarily that weird that your scores didn't match GMATPrep. The standard deviation on the official test is about 30 points and, though they don't publish SD for GMATPrep, I think of it as "almost as good but not quite" - so call it 40 points.
Even that, though, means that you have a 2/3 chance of falling in a 40 point range on either side of your score. eg, if you score a 720, that's saying you have a 2/3 chance of scoring between 680 and 760, and a 1/3 chance of scoring outside of that range.
Given your description of the test (timing was fine, questions seemed easier than you were used to), you probably were making careless mistakes due to nerves (and you weren't aware that you were doing so). Knowing that quant is normally a strength may also have lulled you into a false sense of security, which (coupled with the nerves / stress) also could have led you to make more mistakes than normal. And sometimes we just have an off day or have the bad luck to get questions that are more in our individual areas of weakness.
Unfortunately, you can't see your question results from the official test, so we can't know for sure. The only thing we do know is that you weren't performing as well as you normally do on mid-level questions (mid-level for you), so the test wasn't offering you even harder questions.
Go back and study all of your foundational stuff in quant to identify any little holes or areas of weakness that can pull you down. If you make your foundation very solid, you will minimize careless mistakes and, if you can get most of those questions right, that will lift you to the harder questions in the database. (And study the harder stuff too, of course - but don't neglect the foundational stuff.)
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Stacey Koprince
GMAT Instructor
Director of Online Community
Manhattan GMAT
Contributor to Beat The GMAT!
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