Actually, it does. There can be only one true reason for why something happens. If the true reason for an in-game event is that it makes for a better story, then the reason isn't due to purely in-world causal processes. The two are completely mutually exclusive.
As a player, you can't direct a character as though they were some character in a play, while also inhabiting their mindset and making their decisions as though they were a real person. It's a logical impossibility. Either you're role-playing, or you're story-telling. If you think you're doing both, then you're wrong.
These claims are false.
The reason things happen in the real world is real-world causal processes. That extends to events of authorship, incuding someone saying (eg) "There are Red Wizards in Thay - shall we go check 'em out?" It also extends to imagining things, which is a real world event.
The reason imaginary things "happen" in imagined worlds is whatever we imagine them to be. Because it makes for a better story, I might decide to imagine that Conan's jailer is the brother of someone he once killed, who recognises him and tries to kill him. (This, or something close to it, happens in The Scarlet Citadel.) But that doesn't mean that my real-world motivation is part of the fiction. The fiction is whatever I imagine it to be, and unless I'm choosing to be self-referential/fourther-wall-breaking, I don't need to imagine my motives as author as part of the fiction.
As far as the activity of roleplaying is concerned, there is a whole body of work, both commentary and RPGs (mostly originiated at the Forge, but not confined to there), that identifies and explains techniques whereby, at various stages of the overall process of play, various authors can have various responsibilities for establishing various bits of the fiction.
For instance:
* The player, at time A, decides that his/her PC has a brother. The player does this because s/he thinks it will be interesting.
* The GM, at time B, decides that the next person that player's PC encounters will be said brother. The GM does this because s/he thinks it will be interesting.
* The GM, at time C, says to the player "As you come round the corner fleeing the pursuing guards, you bump into someone. It's your brother!" The player then decides what his/her PC does in reponse to that situation by inhabiting the mindset of his/her PC and deciding as if the PC were a real person.
What I've just described is not logicall impossible. In fact, it - or things like it - happen quite routinely in RPG sessions the world over.