Careful here - you're not competing against the other people taking the test that day, you're competing against the test itself. The curve has been worked out in advance by the AAMC. So you don't have to worry if "lots of really smart people take the test in April" or whatever.
Remember that the AAMC works very hard to create a test that can be compared between people, across test administrations, across a period of 2-3 years. The whole point of a standardized test is that it's been standardized.
In a sense you're right, that it's "curved" against a particular scale, but it's not scaled against the people sitting in the room that day.