Because it needs to be the same person arguing the one point in order for the point to stand?
If a good chunk of the staff can't do the main expectation of their job it's a systemic problem that speaks more to the failure of the entire chain. Telling a 20 from a 10 from a 5 is basic counting.
That's not hostility. That's basic observation from someone who has worked in retail for years. It's basic understanding that you are responsible for your cashier. If you fuck it up, you pay the difference, if it's a constant issue, you're prosecuted.