How can it be distiinguished? You make this statement that there's a difference between creating an encounter map and pre-authoring setting details but don't actually provide an argument for the difference. To me, the only difference would be degree, not kind.
Here's one example of the difference: I turned up to my first Traveller session with four worlds generated (each world takes about 5 minutes; I'd rolled these up in the course of re-familiarising myself with the world generation mechanics).
These weren't elements of the shared fiction; they were notes on a piece of paper.
In the course of the session three of those worlds were introduced into the shared fiction - as a stopover; as a destination; as a further world where a pathogen was known to come from.
The other world is still there on my bit of paper in case I need it. It's not part of the shared fiction.
Here's another example of the difference: I turned up once to a 4e session with my copy of MV2 - Threats to the Nentir Vale. That's prep (all these monsters statted up, with varying degrees of backstory). During the session, one of the PCs started a ritual sucking chaos energy out of a defeated hydra-like fire drake. Something went wrong, and I wanted some creatures to be summoned from the mountains by the out-of-control chaos energy. I flipped through the MV2 and founds mooncalves, which seemed to fit the bill. So now it was established in the shared fiction both that (i) mooncalves exist, and (ii) some of them are here, now.
Here's another example: In my BW game I statted up a dark naga, following the guidelines for creature design in the Monster Burner. I wan't 100% sure how I was going to use it. Then, around the same time, I got a copy of the "Paths of Spite" (then a pdf online; now part of The Codex) and wrote up a dark elf NPC. I wasn't sure how I was going to use that NPC either.
Then the opportunity to use the NPC emerged - as a thorn in the side of the PCs in the context of failed Orienteering checks to reach the waterhole at the foot of the Abor-Alz after trekking across the Bright Desert. And this also then gave some context for the dark naga - as the master of the dark elf, whom the PCs ended up meeting in a cave loosely inspired by B2's caves of chaos.
And another example: for my Cortex+ Heroic Fantasy game I statted up a crypt thing. I knew I wanted to use it, but wasn't sure how. Then one of the players - who was in a dungeon tomb - established a Secret Door asset, which led to a hidden part of the dungeon. That was the perfect opportunity to use my crypt thing.
Those're are instances of prep which don't establish anything about the shared fiction prior to play; but they certainly make my GMing easier.