What im guessing is that at some point, they extracted a "slice" of the game to a different build.
Then a team at FROM worked on that Demo build while the rest kept working on the main game, at this point, both versions were missing the stuff missing from the retail version.
The team working on the Demo build, added tons of details, and activated the lightning/shadowing stuff from their new engine, made it stable and finalized worked on it to be showed to the press.
But that stuff never made it back to the Retail "branch" if you will, for some or other reason, maybe that team came back to optimize code, or finish other areas, etc.
Its the only reason that makes sense to me, because what artist would knowingly take out some of the stuff the GIFs in the previous pages show, stuff like the water splash of the boss when he jumps, how expensive can that be?
Also worthy to note that they made this in the past, The Painted World of Ariamis in Dark Souls 1 has tons of minute details that arent so prevalent in the rest of the game, because that was the first area they made, so it was kind of a tech demo/playing ground for testing gameplay and the engine.
