The example you give of Ron being King Arthur's descendant is an extremely irrelevant fact, and so I'd be inclined to say "okay, sure, I'll accept that this is

The example you give of Ron being King Arthur's descendant is an extremely irrelevant fact, and so I'd be inclined to say "okay, sure, I'll accept that this is true, but so what?" if it was pronounced. It'd be canon or close-to-canon, but it has no actual use to explain or predict within the story. (Unless she'd said it before the series was done, I guess, in which case it'd have caused a bunch of speculation about whether Arthurian myth elements would suddenly become important.) This would create a situation where there's a canon / near-canon fact about a character... that's extremely unlikely to come up. So unlikely that I suspect that the only reason it might have impacted any fan works at all is that there's almost a million Harry Potter stories on fanfiction.net, so there's probably at least a handful that have used "sole living descendant of King Arthur" as a plot point. This gets back to my take that "X is canon" and "in my fan work Y is what's true instead" are not positions that're in conflict with each other. We can accept that the canon story has a bunch of canon background information that informs how it happened, but that's no reason not to change it if you want to tell a different story. Some not-stated-in-text background details do turn out to be deeply rooted in character motivations / setting functionality, and so are harder to rip out than others. Now, on to some rambling about how a story and a contract are different, and the conservation of detail in stories... A story that spells out absolutely everything about why the world is the way it is and exactly why every character does everything is probably not very well written. The author should give you enough details in story that you're willing to believe that there must be a perfectly good explanation for things, even if you've been not told it for any specific event. It's equally valid, of course, for the author to have elaborately planned out the history and structure of their world in advance, or to just make up non-contradictory new facts whenever they need to use them. Just so long as the right vibe is given. I'm generalizing a bit here, because exactly how true this is can be a matter of taste. I personally enjoy the sort of sci-fi/fantasy book that drops you in with weird words that you have to figure out from context and protagonists who won't consciously-explain anything that they already think of as "normal", so I'm clearly biased in one direction. (To give a popular recent example that doesn't go too hard on being weird, it takes a fair while in Brandon Sanderson's Way of Kings before you realize what the fuck is going on with "grass" and "hounds".)