I think 1SD puts you in the 75th %ile (1 SD is 50, 2 SD is 95, 3 SD is 97, as I recall). So 50 % of test takers score within 1 SD of the mean. That leaves the other 50 % outside of those scores, and you can assume that half of those (25 % overall) scored below, and the other half scored above. Does that make any sense at all?
Also, as far as comparing %iles on MCAT and USMLE, keep in mind that the pool is different. The average MCAT for matriculants is hovering around 30-31 now, which is (I think) around 75 %ile. That might be why someone said that a 240 is the same as a 31-32. However, the bottom 50 % or so of MCAT takers didn't go to medical school, so maybe that puts at the 50 %ile mark, leaving you with a 217. That's if the %iles correlate, which I don't think they do. (I'm making a lot of assumptions here, but I think it can be an interesting exercise).
The boards, as someone said earlier, are much more based on what knowledge you do and don't have. They give you a situation, and you figure out what's happening. The MCAT is about extracting information you've never seen or thought about and combining it with basic science stuff and coming up with answers.