Arm Wrestling?

My girlfriend and I have arm wrestling matches occasionally. Most of the time I am able to get an advantage on her. But then, she goes into a "defensive" mode where she will bend her wrist backwards, and most of the time I can't advance her arm any further. On 2 separate occasions she managed to get the advantage while in this "defensive" position. I don't feel as if I'm being overpowered, but most of the time she is able to hold me off. I've always wanted her to do good against me, but I wanted her to win with both of us trying. At first I wondered if I was really trying, but then when we put a time limit on it, or a bet, she was still able to hold me off. The only way that I win is when she just stops trying. Do you think that I am really not giving it my all or is it that she is just better at arm wrestling than me or possibly stronger, since she says that she really doesn't like arm wrestling?