* The specific practice test: Different practice tests have different difficulty levels and scoring systems. A score of 75 on one test might be equivalent to 85 on another.
* The norming group: Percentiles are relative to a specific group of test-takers. The percentile associated with a given raw score will vary depending on the characteristics of the group used for norming (e.g., age, education level, intended field of study).
In short: A raw score in the 70s or 80s on a practice MAT is *not* directly translatable to a percentile. The practice test should provide its own conversion chart or percentile ranges, or at least state the approximate percentile range associated with those scores. If it doesn't, the score is largely meaningless in terms of predicting your performance on the actual MAT.