Are you looking at what specific questions you are getting right vs. wrong?
Without any additional data, it would appear to me as if you are not consistent across the question types, and so, depending on the question set you receive, your score varies pretty drastically.
There are less than 80 questions on the whole exam, with five types.
Let's pretend you are a killer at SC and CR, but your RC is terrible. Well.... you're only going to get a few RC questions. If you are rolling at the time you get them and they are tough RCs and you manage to get say 1/2 right... you'll end up with a pretty good score. If on the other hand, you are struggling when you reach these RCs and are given relatively easy RC questions and STILL get them wrong... then low score for you.
I had some of this inconsistency myself and did quite a bit of analytics to determine that, for me, it was arithmetic questions in DS that were tanking me, and depending on where they landed in the problem set my score varied considerably.
I'd suggest trying to understand your relative strengths/weaknesses in question type and content area (for quant) and see if that helps.
The other note is that the GMAT is a highly variable test. Let's pretend your "true" ability is in the 660-680 range - well, you're not going to be able to answer ALL of the 630-650 questions correctly and NONE of the 690-720 questions incorrectly. Ability isn't that standard. The specific questions you receive and your specific ability (and timing) to handle the question determines your score. Many people experience score variation - that's one of the reasons why folks encourage people who are dissatisfied with their official score to retake, as just getting a different problem set can result in a different (hopefully higher) score.
Apologies if this was just babble - trying to be useful/clear here.
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