LOL - I have no clue why we can't be simpler. But complicated keeps things interesting, eh?
If you're sure she's never had better, then maybe she's been talking to her friends about you and comparing stories and they are telling her you're only average.
If it makes you feel badly, then no matter what her motives are for saying it, she shouldn't be. It sounds like you're half-joking about it bothering you, but if even part of it does, and if she knows that, she shouldn't continue to do it. I'm guessing you're both rather young - these threads always make me feel so old and jaded. I haven't played games like this in 15 years - I guess the best thing I can tell you is as women mature, they aren't always so difficult.