Playing devil's advocate here but perhaps some of the quizzical looks you get is due to the fact that the "girl" at the register hasn't encountered anyone giving extra in order to receive a specific change amount. After dealing with thousands of interactions with people paying with CC's, their phones, or paying in whole bills out of nowhere she's presented with a new scenario. Perhaps she thought that she was being nice to the customer who obviously made a mistake by giving the extra dollar when a $20 bill would've covered the amount. Also many modern registers have hot key buttons for payment with $5, $10, $20, $50 and putting in a specific cash amount is something that is rarely done so it could be a learning experience for the salesperson.
Going back to the rant thread from yesterday about poor service, maybe no one has ever explained the concept of paying a higher amount to get specific change to her. Maybe she was never trained in this concept by a manager, or her parents, because to adults who have learned this skill it's second nature - but it's new to her. We were all young once, though for some it's been so long they've forgotten what it's like when you're just starting out and having new experiences placed in front of you.
But I doubt this will strike home and I'm sure another "haha young people are so dumb" thread will come along tomorrow.
