Following on from my other two comments, I don't think it's completely arbitrary, I think the way it works often follows some vague guidelines people have absorbed subconsciously. That is, I think most of the time, books succeed in subtly conveying what is possible and what isn't (and which things, if the protagonist poked at, would reveal the holes in the worldbuilding).And I always LIKE trying to come up with consistent underlying rules for the worldbuilding. But I think trying to justify the magic usually doesn't actually help. Like, for the story to work, you need to know what things are easy, what things are possible with a lot of diligent research or luck, which things are sufficiently touch-and-go it's reasonable for two mages to struggle rather than one just winning automatically, etc. And you need to know in a bit more detail than is actually explained in the book to keep things consistent.But I think that usually, trying to build up all that from first principles, rather than "one step removed from what's in the book", doesn't usually help. I'd love it if it did. I love books where someone takes a small number of physical differences and extrapolates society, etc from it. But I think it usually doesn't work. It'd be like saying explaining electrons and expecting someone to extrapolate all of consumer electronics. They might guess which sorts of things might be possible, but will never easily guess the specifics of "all books from the last 10 years are on google, before that it's hit and miss" or whatever. That takes more worldbuilding, which wouldn't be much worse if instead of electrons it was just "magic".