So, originally my thought was to to discuss canon more generally, and why it is similar to discussions about legal precedent, but then I decided to switch it up to examine FR and GH. So I didn't make the full argument, given that I had already written so much.
Briefly, though, most arguments by fans about canon are similar to lawyers arguing to a court about precedent. That's why I think that the FR Wiki "hierarchy of canon" is so interesting.
In effect, canon is whatever the brand holder decides it is. And the brand holder (much like the highest court) can simply change its mind at any time! For example, what ARE Gorn? We had a long-standing precedent from TOS, but (arguably) that precedent has been overturned or distinguished in Strange New Worlds. What was canon (precedent, binding) is no longer canon, exactly.
Fans arguing about canon are arguing about what is really precedent. About what is official (binding) and what isn't. Fans of Greyhawk, for example, might argue that the only "real" precedent is Gygax's own words, and others might argue that its any thing official from TSR/WoTC. Or, even more particularly, fans might argue that it's only Gygax's own word up until WOG (the boxed set). And so on.
This brings up two related issues-
First, since the brandholder can change precedent at any time, I think that a lot of fans argue that some things can't or shouldn't be changed. This echoes the "reliance" issue with precedent. AKA, "FR has always had Elminster, so you can't retcon him out of the setting." This is the push and pull between complete control over the canon, and the need to maintain links to what has come before that people love and rely upon.
Second, I would argue that settings are generally different than the canon in fictional universes (LoTR, Star Trek, Star Wars, etc.) for the simple reason that every table has different adventures, and so every table will necessarily have different events and different "canon" than what has been published. For example, I am quite sure that more than one table has had its party kill off Drizzt, and yet he endures in the official canon.
Hope this helps.