I think some women fake it not to insult their partner but rather to get it over with. That sounds awful, i know. Take into consideration the woman's point of view. He wants to go at it and she thinks to herself 'ok i could go with this..' everything is fine and dandy and then she's not feeling it quite as much as he is, or right at that point he wants to bang the junk out of her and she's starting to dry out/burn, or she's turned off.. whatever the case may be.
I'm not saying that its OK to fake an orgasm. I'm just saying its EASIER that way sometimes.
(Oh and for the record I don't fake orgasms with my fiance)
Just talk to her and ask her why, maybe knowing will put you at ease. Maybe not. Anyway.. i agree with lurkette.