I think the percentile shows where you stand among the limited number of people who took that same test. The numerical scores are probably based on the number of questions you got right/wrong. The 0-30 score is how you did against the test. The percentile is how you did against other people who took the same test.
Most statisticians would argue that the percentile is the better number to judge an applicant. But it is the 0-30 score that schools can advertise so that's probably what matters to them.
For a 2009 test, a 20 AA would mean a 89.7 percentile but for my 2012 July test, a 20 AA was a ~93.5 percentile. However, a 20 AA's percentile for one person is a different 20 AA's percentile for another person who took the same test because there is some leeway for the number of questions correct that will get you a 20 AA but varied percentile.