What I think is this:
Thanks to some funky quantum math (don't ask me) It seems like every single possibilty, must exist - that's the multiverse. Parelel dimensions, when you had a three way with Christina Ricci and a young Tia Carrere, while after getting back to shape from your affair with Doomsday and the Six Million Dollar Man. Things like this, obscure as they are, still possibilites, so in one of the worlds must happen.
(Or not. Whatever. I work with photoshop, so don't ask me how the multiverse theory works.)
I think, the way our world works: if someone writes a story, or a "fable" enters the public's conciusness, their world will get closer to us, and make crossing possible, or even likely. Or even better, our world acts as a central hub, and our stories are organizing these worlds around us (Hence the shared realities of the Homelands.)