...If she thinks she's straight, why did she sign up to be a lesbian call girl?
Well, to play Devil's Advocate, the opposite is not unheard of.
I remember, in the movie "Working Girls" by Lizzie Borden, the MC was a lesbian in a committed relationship with another woman. She and her wife lived together and were very happy, they were even raising a child. Everything looked okay with her life. Her job, however, was working as a straight prostitute in a cozy middle-class brothel. So perhaps the other way around is not that far-fetched?
No, but, honestly, I think the real answer is probably that she thought that being a lesbian call girl is more decent than being a straight call girl. After all, gee, dicks are not involved!