Suppose you have a test which is a decent proxy for how well someone will do a job. The median person currently doing the job scored 85 and their range is 70-99. If you put someone who scored a 4 in the job, people will die almost immediately. If you put someone who scored a 50 there, people will be at a higher risk of death and you'd be better off passing on that candidate and waiting for a better one. From this we might come up with a threshold of 70 for the minimum score and call this "qualified". Then if you have to fill 5 slots and you got candidates scoring 50, 75 and 95, you should hire the latter two and keep the other slots unfilled until you get better candidates.

But if you have to fill 5 slots and you have 10 candidates who all scored above 70, you now have to choose between them somehow. And the candidates who scored 95 are legitimately expected to perform the job better than the ones who scored 75, even though the ones who scored 75 would have been better than an unfilled position.