Because of the guilt trip, mainly. However - I think I read this the other day? - Christine is all "I CANNOT LIE TO YOU, I'M SORRY" and her lies are harder to keep up with Raoul. With Erik...she either lies better because lives are on the line (as well as her freedom) or she doesn't have that same attachment though she says her lies were as horrible as his face was. (And as for leaving: Raoul gave her the offer and she accepted it, saying she needed only one day. Because of the fucking guilt trip again...) But that's just me. That's the problem with this book/fandom/series/whatever. It's really about interpretation. :) Is this a good time to say that if you think either Lotte or Christine are off, do tell me?
...And my opinion is colored because Lotte is screaming.