in defense of cashiers(of which i am one, and i do know how to make change in my head) when you are busy and the cash register does the work for you, you learn to not pay attention to numbers you don't need to remember.
example. you buy 5.50 worth of stuff...i tell you what you owe and you give me 10$..I HAVE to punch in 10.$ and when i do the 5.50 amount leaves the screen and your change comes up....now when I have 50 transactions in 15 minutes I simply can't remember what everyone's tally came to so I simply try not to retain that number. Now if you gave me 10.50 it does stop me for a second because I have to get the number on the screen out of my head and think for myself for a second:)
Its not that I can't do it its that I have to stop mid transaction and make my brain think differently.
Don't punch in $10 tendered if the customer gives you $10.50
SD