Whatever happened to "The customer's always right"?

Last night I went to the store to get groceries. The total came to $95.01. I had some twenties and a couple of fives. So I handed the checker five twenties and a five, then told her to wait while I got her a penny. When I handed her the penny, we had this conversation:

Her: Your total's 95.01
Me: Yeah
H: You gave me 105.01
M: Yeah
H: I don't need this. [tries to hand me back the five dollar bill]
M: Yeah, but I don't have a ten. [Looking back on it I can see how this confused her even more, but at the time I was just getting frustrated because she was arguing with me.]
H: [stares at me blankly, then tries again] Your total's $95
M: Yeah, I know
H: I don't need this.
M: Fine, just give it back to me.

And I took the five back from her, so that she could give me my change with another five, so that I now have two fives instead of one ten, which is what I wanted.

And while I totally understand that this is a stupid, trivial thing, and now that I've had time to think about it, I also realize that virtually no one in this state has ten dollar bills anymore, since all the ATMs only give twenties and fives, I still don't understand why she didn't just punch in 105.01 on her cash register and give me my change instead of standing there arguing with me about it.