One thing I like in my games is consistency. What works now will work later, given identical circumstances. The results will be believable and acceptable, and if they don't repeat it's because the circumstances only appear identical.
But I'm not willing to concede the self-consistency of my game to consistency with external sources. The context of my game world is set by what I've written, selected from sources, and then used or demonstrated in game. In other words, my decisions trump books and in-game usage trumps even that.
But if I'm basically changing stuff, where do enterprisingly players turn to find out what the in-game ground rules are?
I think that just like external consistency (matching the books) and internal consistency (matching prior play), you have external sources of information and internal sources of information.
What do I mean by external sources?
Things from outside the game world, not handed to them during or before play by the GM or specifically designated as correct and in use.
- monster manuals and rulebooks in general
- knowledge of folklore
- information from previous campaigns of mine
- video games
- fiction of all kinds
- knowledge of just how many pirate/pygmy/orc/zombie/genestealer/etc. minis I have and suspicions I might use them this game
- wild-ass guesses
These are hit or miss in terms of value. They might be right (yes, the trolls in my game aren't so good about fire) or wrong (the wights in my game paralyze you). Relying on this is risky.
What about internal sources?
Things from inside the game world, either learned in play or handed to the players by the GM.
- out-of-game explanation
- evidence of their own play
- sages, wise men, etc.
- written sources
These are also risky. The first one is the least risky. In fact it's generally infallible. "Are we handling fireballs the same way this game?" or "Can I run a holy warrior?" or "Are all barbarians in my tribe SM+1?" or whatever. The answer is certainly trustworthy, although it might not be helpful.
The second, evidence of their own play, is the next least risky. Of course if their conclusions are wrong, or facts misinterpreted or not understood correctly it still can be. But it's also the hardest to get.
Sages, books, and rumors are all going to be of varying degrees of accuracy. Mostly degrees of inaccuracy, with some being flat-out wrong and others being mildly wrong or just misleading. Generally what they hear at least has some in-game value, in that they're hearing what some other people believe to be true. It's information that I as the GM am willing to cop to, to hand out, to hand to you to see what you do with it. It's information in play, out on the table, and not something that may or may not even exist.
That's what separates the two - one has automatic in-game merit, good or bad. The other may or may not have in-game merit at all. It might be in the game's canon, but not in the campaign's canon. The internal stuff, well, it's in the campaign's canon, it just might be true or false. But at least it's there.