It feels like, Kaye is acting less like her original self because despite being studious, we did learn that the girl had fantasies of being a magical girl, that she can now fulfil. MC on the other hand, despite being the one with the book, found the plot very ridiculous and wasn't really enraptured by it.
So one sees it as a second lease on life where she can be a magical girl, and isn't really thinking of her original life, while the other one wants to go back if she can, and is also worrying about how to not die in 7 years. At least that's how I'll explain to myself the apparent differences in their thought-processes despite mentally being the same age.