The standard deviation of our test is about 50 points or so. In addition, some people get very nervous in the official test setting and underperform because they are so anxious.
You should also take a GMATPrep test - from the people who make the official test - and see how you do there. These tests are generally considered a little easier at the high end than the real thing because, though they do use the official scoring algorithm, the question pool is much smaller, which decreases the validity of the test and makes it easier for high scorers to perform well.
But, in general, you should feel good about where you're at!