well.... kinda....
its based on test versions. When I called ADA, the guy who answer really seemed to understand why %-ages are different between the same scores (example some 21s are 91% and some are 98%). What I gathered from him was that there are always MANY test versions floating around, he guessed around 50 different test versions floating around, but the major problem all tests have in common is that they are graded on a "semi-straight" curve (keyword = semi).... Meaning (for example) if you miss 5 questions from the biology, that automatically takes you to a 20 IRRESPECTIVE of what test version you have, if the 20 happens to be a top ranking score (like above 95%+) then it means this test version is a little skewed and needs re-adjusting so the system designates 6 wrong for a 20 (instead of 5).... BUT THE PROBLEM IS, many people took it at the older difficulty (so they missed 5, landed a 20, but was considered 95%).... eventually after many people took the same version, the system adjusts itself BUT NOT BY THAT MUCH.
there is way more he explained to me but I don't know how to put it in words....