[ Gender & Women's Studies ] Open Question : Why do girls lie about how good their boyfriends are in bed?

I just don't understand why a girl would lie about how good her BF was in bed. I am one of the guys who legitimately tries to please his girlfriend. How would I know if I am doing a good job If she is being dishonest about it? What is the benefit of lying to your boyfriend about how good his is in bed? BTW, this has nothing to do with my GF. We actually have a very open and honest relationship.