It seems to me that this is quite similar to the moment in RPG's when a designed scenario full of possibilities switches to a narrative story, at the point in time when you actually play it. That is, "scenario becomes story" when the dice hit the table and we find out what actually happened. Prior to that moment there are endless possible outcomes -- a statistical space, really -- whereas afterward there is one known outcome which we then seek to explain as a "story". Story generally exists only after the fact, not before play, in RPG's. *
We might more broadly say that this occurs in any live performance: theatre or music, for example. Technically many possible exciting things can occur which we only know about for sure after the fact. However, this phenomenon is several orders of magnitude easier to see in RPGs, which has such an overabundance of possible outcomes (in conjunction with both player choices and random die-rolls), as opposed to theatre or music, where the script and rehearsal set a relatively narrow expectation for what should happen in the performance.
* Note: This is a heretical interpretation on my part.