If you read some of the success stories on this site, you'll see that many people who score 750+ have taken the GMAT more than once and have put in a lot of time and quality, targeted studying. A lot of them will have first scored in the upper 600s before adjusting their study pattern and tackling the exam again. There is, of course, always some innately good test taker who scores 770 after only studying for a few weeks, but overall, I think the 99%ers are just normal, smart people who have worked hard to master the content and the strategies.

If you play with GMATPrep, you can deduce that to get a 750+ score you have to make very few mistakes. It is also a question of luck: your score will suffer more if you answer incorrectly several easy questions in a row in the begining vs several tough questions, not in a row, at the end of the section.

The superhigh scorers make very few mistakes because:

(1) They know the material well, they have practiced many problems, or they didn't practice that much but had a good background to begin with.

(2) They know well the pitfalls that the test sets up for high scorers. From every problem they got wrong, they extract a pitfall and then learn to watch out for and recognize that pitfall in later problems.

(3) They have fast thinking, they are focused, they mark key words (which is also EXTREMELy important in high level quant problems) and they do NOT check their solution 1000 times for the fear of being wrong. Some people may be genetically gifted to do that naturally. For the rest of us, with foggy thinking, lacking concentration and obsessive-compulsively checking the solution at least 3 times to make sure it is right, there are stimulants that can help override mother nature and make us think fast and focused.