The fiction represents what is stated to be true about the situation the player characters are in.
In a sense, the GM is subject to the fiction as well, or to make the point more pointed, IMO the GM is subject to the fiction as well. There are different theories about how to GM that disagree on this point. In my opinion, if you have established that there are eight orcs, you shouldn't suddenly decide that four more orcs show up because the fight is going badly in some way. You should only have those four more orcs show up if you know from the fiction that they were already there, and those orcs in turn would now empty some other location you already knew of in the fiction.
Since the fiction is something I established, it's a bit weird to speak of it overriding me, but in the sense that once established I also have to treat the fiction as real and react to it and am limited by it, then yes I think the fiction is overriding me. I might not want the bad guy to die without providing dramatic resistance, but if the fiction is establishing that that is happening, I can't or at least shouldn't decide to override the fiction to get what I personally want. Afterall, I largely was responsible for establishing the fiction in the first place, presumably having decided that that is what I wanted. If I can mid-scene change the fiction because I have decided I'm not getting enough of what I want, then essentially I'm assuming sole authorship of the story.
This is my perspective at least. I know of quite a few GMs that publicly argue for a total illusionism stance where the GM at all times is overriding the fiction and even ignoring the rules for "the good of the game" while pretending to the players that there is some sort of process going on that isn't continual fiat. In this GMing stance, the GM owns the whole story and presents it to the players while giving the illusion that the players interaction is meaningful.
The really odd cases are well respected GMs that don't realize that they are running their own games from a total illusionism perspective, and have in fact tricked themselves into believing in the illusion and who think they are leaving the story up to the players despite hard obdurium walls and heavy use of fiat to steer scenarios to the desired result. For example I saw one GM describe what amounted to an unbeatable trap that the players were required to fall into, which he ran according to the super harsh rules he'd established and then, just as the situation would have resulted in a TPK he overrode the fiction to produce the result he wanted all along. This is a guy who writes up very elaborate rules and procedures to establish the fiction, rules that statistically guarantee the futility of whatever the players doing and then uses this as an excuse for hard fiat for the good of the game as the ultimate end of the scenario with no apparent self-awareness that the elaborate rules he set up more or less guaranteed either a TPK or else a GM fiat intervention while bragging about how crafty his scenario was.