It's pretty much as Alex said. Oblivion and Purgatory mirror Dystopia proper, and the last two worlds are spiritually connected to it. Dystopia proper is the only one of them that has a set of stars to look at in the night sky. Character origin is important, because you're effectively adding something to the world with each post, and I personally, as the original creator of Dystopia, want it to remain clean of influences from that of alternate universes and the like. The easiest answer is that it's my personal preference. Again, harkening back to DC comics: when you've jammed so many connections to alternate universes into one single universe that you have to make one long running plot where you cull and separate these universes, you have a problem.
The plot you sent me honestly seems just fine. I'd certainly play a character in it. Aside from mentioning multiple worlds, there's no indication of multiple universes outside of the established few. That being said, the same thing you asked applies: what does the character's origin matter? If you don't much feel like reinventing an old concept, it's easy enough to just change the details of birthplace and a few place names, and the same fantasy character fits into just about every other fantasy world and roleplay.
That being said, I have no idea why that thread died off right as soon as you joined it, but I know it had nothing to do with character origin. There were some long standing issues between players and creative differences in Kalmarden that sadly never got ironed out.