Honestly, while it hasn't been explicitly stated as such, I've been leaning towards the idea that each "world" is actually a separate dimension/universe for a while now.
1. There are no less than 3 Londons in KH1: The Dalmatians' home world destroyed by heartless, the home of Jane & Clayton as mentioned by them on-screen, & the Clock Tower area in Neverland (also mentioned on-screen by Wendy). Maybe one could make the argument Jane & Clayton are from the same London that's in Neverland, but to reconcile the Dalmatians being from that London too when it's outright said their world was destroyed?
2. Similarly, DDD has 2 Frances, 1 of which is populated by talking animals & the other isn't.
3. All of the worlds used to be 1 big world originally. So then, uh, were aliens part of that 1 world too? Because we got aliens in KH already & boy howdy does Deep Space raise a lot of questions. Like Planet Turo is a part of the DS world. So is that an outright confirmation that planets & worlds are considered separate things? For that matter were there 3 Londons & 2 Frances in that 1 world as well?
4. According to Nomura (I think), worlds have their own laws of magic & death. & time moves at different rates in each world as well (stated in DDD). This makes more sense if each world was its own reality rather than all of them sharing a single reality.
On general principal there should theoretically be such a thing as Disney films that can't be included into the KH series because they don't mesh with established canon. But Square's never been 1 to let that get in the way of throwing in worlds willy-nilly. As such, the official explanation for the existence of Disney worlds doesn't really hold up well & is probably overdue a retcon or two.
