bigs the idea is that first of all, it's not the same pool of test-takers every time. Johnny, Jill, Mary, and Steve are taking it in June. Johnny, Mary, and Steve are happy with their scores, but Jill decides to retake in October. Karen, Mike, and Angela are also in sitting in October. IF you're one of the people who believes that the curve is set after everyone takes the test, than you'll think that Jill received a score that was determined by how many raw points she earned in comparison to anyone else. In this case, even if Jill scored below median in June, it doesn't mean she'll be below median in October. What if the pool of test-takers is less prepared in October and what if Jill studied her heart out and jumped up 17 points and went from a 153 to a 170? Therefore, someone whose percentile was probably around 55 (I don't feel like looking it up for real, but the exact number is not important). Suddenly, she finds herself in the 97th percentile. That is what we're talking about. Just because someone scores below a 160 doesn't mean they don't have the potential to do so. Any of Jill's fellow October test-takers could also re-sit in December after ample self-study and could end up being the next Jill.
On the other hand, if you're one of those people who think that the curve is set before the test-takers take the exam, and this is probably the case, that means that yes, more than 20% of test-takers can fall above the 20th percentile, because the raw number of questions one would need to get right would already be determined. Let's say that number was 28. If more than 20% of test-takers get 28 or fewer questions wrong, then you would have more than 20% of test-takers above the 80th percentile. Someone already pointed this out earlier in the thread.
And once again, we're talking about individuals. Each reasonably intelligent individual has the power to reach the 80th percentile, even if they throw in the towel before they reach it.