Eg: Person A from Literature A is capable of magic, this magic is gathered from praying to a goddess. Throughout the books there is no deviation from that that doesn't make sense (Person A discovers that they are in fact travelling with said Goddess and just asks that goddess to do things from time to time instead of casting a long intricate spell.)
Person B from Literature B is also capable of magic, this magic is gathered from reading a book. Halfway through the book the character loses their book and starts pulling magic out of his or her ass. You no longer have suspension of disbelief if you've worked to establish that character as absolutely requiring the book.