A bunch fail on their first attempt, fewer fail it on subsequent attempts because they know what the test looks like and how to prep for it. If a test has a 50% pass rate but some of the test takers have taken it before and are more likely to pass, then the first time test takers will fail more than 50% of the time. Basically, the people who are on their second or third attempt will skew the results against first time test takers, because they’re eating up all of those “pass” spots.