The scores are standarized and distributed, when X number of points corresponds to a certain number of standard deviations. That is specifically what standardized means.
You are basing your reasoning on the assumption that the NBME recognizes significantly better scores with significantly better three digit scores, even though it would severely skew the SD. As you get to the top or bottom of this distribution, you should not expect to see drastic jumps in the scores, I dont believe, since they do try and keep it standardized. So, a 290 is nearly 2 SD above a 260, but you cant physically quantify that in a score, it just doesnt make sense. Why would you assign a score that much higher to someone when you have standardized the test. If anything you would see a smaller drop off in score relative to questions missed (290 = 0 questions missed, 280 = 10 questions missed, rather than 290 = 0 questions missed, 280 = 1 question missed)
I dont think its possible for the high score on this standardized test to be any higher than 275-280