I just Beat The GMAT!

No, the score is not a direct average of your percentiles- that wouldn't make sense to do, statistically. You can think of it this way- say you score in the 90th percentile on the Quant *and* 90th percentile in the verbal. If scores are randomly distributed, the probability is 0.1 that you'll score that well on Quant, *and* 0.1 that you'll score that well on Verbal. This is now a GMAT-style probability question- the probability that you are above 90 in both is (0.1)*(0.1) = 0.01. That is, approximately one percent of test-takers should score above 90th percentile in both sections. 81% will score below 90 in both, and 9% will score above 90 in one, below in the other. Now, that doesn't mean you should be in the top 1 percent if you score in the 90th percentile in both, because someone might be in the 99th in one section, and in the 88th in the other, and deserve a higher score than you. As it turns out, if you are in the 90th percentile in each, you will end up in the 96th percentile or so overall. But the point is, if you're above average in both sections, you'll be even more above average in your overall score, and if you were, say, in the 90th in each section, you are part of a very small group of test-takers. There are statistical principles you could learn to understand exactly how this combination is performed, but I won't get into that here.