Yeah these are just standardized values that can be used for general grading of practice tests (topscore actually uses the exact grading rubric from the ADA website). If you look closely you'll notice that even if you get all of the questions correct on some sections you won't get a 30, while on the real test those people who get all the questions in a section correct will usally be in the 99 or 100 percentile and get a 30. If an absurd number of people are getting perfects on the same section then the highest possible number to get on a section won't be 30, it'll be dropped to 29 or 28 but it will still show as the 99 or 100 percentile.
Wow that got pretty convoluted, hope it made some sense.