I think I can explain some of the weirdness in scores on the math SAT.
According to a quick search, the SAT math section consists of 44 multiple choice questions, and 10 grid-ins. When you get a multiple choice question wrong, you lose points. When you get a question right, you gain points. When you don't answer a question, you fail to gain points. When you get a grid-in question wrong you don't lose points, but you fail to gain points.
If we look at the distributions for the top math scores from Val's link, we see that 770 and 740 are more popular scores than 760 and 730 respectively. This is most likely an indication that answering 1 multiple choice questions wrong results in a score of 770. Skipping 1 multiple choice question, or answering 1 grid-in question wrong probably results in a 780. I still haven't figured out how a score of 790 is possible, so my answer probably isn't entirely accurate. But following my logic, answering 2 multiple choice questions wrong probably results in a 740.
At any rate, I agree with Val's initial assertion that the SAT isn't a very good IQ test. I'm sure performance on the SAT correlates somewhat strongly with IQ, but I think it's measuring other things that are related, such as processing speed, accuracy (carefulness), focus and attention span, and retention. When I take an IQ test, I usually have to think. When I took the SAT, specifically the math part, I simply had to carefully regurgitate problem solving techniques I had been taught several years before. I also had the opportunity to take the SAT before I had been taught many of the appropriate problem solving techniques, and I didn't fair as well. It was quite evident to me in 7th grade (without any exposure to algebra or geometry) that the test wasn't measuring my intelligence by asking me geometry problems with vocabulary I'd never heard before.
Regarding the ceiling, I will say that it's lower than I would like. It would be nice for some of the people scoring perfect or nearly perfect to be able to differentiate themselves from some of the other people scoring perfect or nearly perfect. But I guess that's what SAT subject tests are for.
