I think is has a lot to do with so-called probabilistic models, like, you know, particles are "out there", but we can't tell where exactly they are in a given moment (perhaps, because a "moment" has nothing to do with particles) , so, we speculate that the universe we "observe" is a "snapshot of locations of all the particles", while there are could be many others. But no, there aren't any other. They does not exist, they just does not happen, at least to this particular observer.
Each observer observes exactly one snapshot of the Universe, while every observer "sees" the same Universe from a different angle. But this is the same Universe. Multitude is just a mental flaw, mistaking different perspectives (views) for different "things".
Ironically, this description fits perfectly to ancient notions of gods (and universes). There are many of them, but the mechanism which made them all - a human mind - is one and the same.
I am not a quantum physicist, of course, neither I am a philosopher, but I prefer to rely on philosophy.)