Yes, well, perhaps there was a lot more trust that Peter just violated with Lucy than with Gabriel. Also, Lucy can't actually harm Peter that much, so she doesn't really need to stop herself.

Lucy looks at him, almost with an expression of disgust. At the moment she doesn't really give a shit if Gabriel's secretly a three-headed donkey, and she's even more angry that Peter's trying to distract her.

"Why did you trick me like that?" she asks, completely ignoring his revelation.