I was thinking about this too.
It's notable, when Sora goes into the 100 Acre Wood book, it presents itself as a "pop-up book" in another environment, i.e. the book is making a projection that Sora is interacting with. Also in KH2, Sora goes into that book and talks to Winnie the Pooh, but then Pooh freezes in midair and Sora's knocked out of it by a forcefield. When he gets back in, the exact same scene replays itself, a pre-determined story playing on a loop.
I think it's a microcosm for all the Disney worlds in general. Like, all these worlds appeared chronologically in the Book of Prophecies before they were even actual worlds. Then eventually they were "remembered" as real worlds, but could still be copied to other mediums (memory, data, dream, as YX says). But somehow the 100 Acre Wood stayed as a book, and in doing so, betrays the truth about "worlds".