Numerically Challenged

Friday evening at the Hustler Club outside Detroit, I bought a dancer a $13 drink and gave the blonde waitress a $20. She returned with a stack of $1's and when I counted them there were only 6 bills. I decided not to complain but to make that missing $1 the waitress's whole tip. But I mentioned the incorrect change to the dancer and she immediately called the waitress over and told her about the discrepancy. The blonde waitress looked puzzled, and then she counted on her fingers, realized her mistake, and fished a dollar out of her cleavage for me.

I don't remember when I last saw someone count on their fingers, but that performance convinced me the waitress wasn't dishonest. When I left, I tipped her 20% because I felt sorry for her. She probably doesn't know what 20% means.

Why couldn't the waitress do simple arithmetic in her head? Because she was blonde or because she worked in a strip club or because she was the product of the American educational system?