No, the answer is pretty easy in fact.
Let's say Walmart hires two people with really shitty attendance records, for whatever reason, then they can count on those two to be absent from work, a lot. But on avg. the two workers will be there the equivalent of one full time employee that never calls in sick. Since they have a history of being undependable from previous employers Walmart will pay them less, and should. The drawback is Walmart not being able to predict when these two crappy employees will show up or not, so these employees 'produce', or are responsible for less units than one dependable Costco employee which is why the Costco worker get paid significantly more. That's the concept of marginal utility as applied to wages. But I don't expect you to understand that.