Some people don't score well on tests because they're lazy or not focused, not because they're dumb. Sadly enough, some of the most brilliant ppl I ever knew got the worst grades (and tended to drop out of school here and there). I think it's because, while growing up, they were used to putting in little effort and were so smart the teachers would either pass them, or the tests would be so below their level they would pass them with ease.
Now this says a lot about someone's diligence as well, their willingness (or unwillingness) to put in the time and effort when it really counts. Which of course might not make them a good candidate for med school, but may still make them an effective doctor in the long run. It's hard to tell, but the cut-off of scores and grades is just a rough guideline, because there are so many candidates, it is impossible to interview every single person to decide what was the reason for their poor scores/grades.
Now, on the other hand, sometimes a candidate can "make up" for things by reversing trends (taking grad school classes for example and doing well) or by convincing ppl of their devotion by their activities etc. Sometimes that is not enough, though.
It would be very difficult for anyone to be accepted with a score in the low 20s (or even lower), even with a good GPA. Not impossible, but just very difficult. Difficult to even get through the front door for interview day to argue your case too...