What I'm saying is, in all cases the GM is MOSTLY and OVERWHELMINGLY subject to forces that come from constraints of what material they have available and how they envisage things, not from constraints imposed by the backstory. So, the primary driver in most traditional play (and I've run enough of it to know) is "Hmmm, it would be cool if X happened!" and then X happens. Or "hmmm, I wrote up 33 rooms of dungeon the other day, what chain of events can I invoke that will lead to the PCs going to the dungeon?" etc. None of those are inherently bad, and none of them needs to fail to respect backstory, but they aren't very constrained by it, typically.
Which leads back to my observation a few pages ago, all games are story, all the time. Every time something is introduced into the game, the primary forces leading to its introduction had to do with something the GM wanted to say, for whatever reason. The contrast being games like DW, where a large part of the forces acting on the plot are actually what the players brought to the table. In DW that rises to the extent that every move the GM makes is going to be in reference to something some PC did, or is, or maybe has. The FORM of it is up to the GM in large part, but the salience is based on "That bears on the bond between the Barbarian and the Paladin" or something along those lines. Orcs don't just appear in the corridor because it is cool from the GM's perspective, they appear because they can threaten the Barbarian with death and make the Paladin choose what to do about it. They are Orcs because the GM's front says so, perhaps, but Bugbears would have served equally well if that was what he'd written down (or pulled out of his hat).