I think the realms lie on a spectrum and the ones that seem based on history (like Arthur and Mulan) are the realms that lie closest to the “real world.” The more magic that is in the world, the farther it would be to the real world. I think the closer two worlds are, the easier ti would be to cross over. Since the ones based on history are pressed up against ours, it would explain how it was possible for legends to cross over into our.
a rough example:
Real world –Arthur– OZ (?) — Enchanted forest — Neverland (?) — Wonderland
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