Keep The Change

Having long-ago earned my stripes in running a cash register, I am always amazed when people show such ineptitude in this area. It's really just a big calculator. I don't know how this can be an intimidating or difficult machine to operate, aside from shady math and shifty employees.
I went out to lunch with some friends on Saturday afternoon. As is usually the case, everyone had $20 bills. We didn't feel like leaving a $17 tip on a $43 check, so we needed a little change. One person wanted to put her portion on her credit card. So we slip one credit card and $40 into the check-folio.
When our receipt finally comes back, it says that $14.50 was charged to the card. The change is conveniently absent. This would make the total amount paid $55.50. That would be a 30-something percent tip for lackluster service. I cant help but feel like someone was trying to pull the wool over our eyes on this one. The proper way to have entered the amount into the register would have been as $40 cash. That would leave the rest ($3 something) and tip to go on the card. Why on earth would a random amount of money be put on the card AND all the cash taken? When we confronted the waitress about her math skills, she grudgingly returned our cash to us with the caveat that we could have used the difference on the card to account for her tip.
Wow. Way to be double presumptuous!