For example, take a look at north dakota... according to this website, it accepts 1 out of 5 students who apply. However, that is very misleading because they rarely take people who are not residents of north dakota, which means that out of state people aren't going to even bother applying. The only people who end up applying are not only in state, but are people who know that they have a decent chance of getting accepted. But if you or me were to apply there right now, we would pretty much have a 0% chance of getting in.
Another example: take a look at harvard. There are a lot of schools who are a lot less competitive that have a lower acceptance rate, why? because people (like me) who don't have such high numbers won't even bother applying there. Therefore, their numbers are skewed because they're mostly taking into consideration the people who had high enough grades to think that they had a decent chance of getting in.
hope this makes sense🙂