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Back in Oras when they started the whole multiverse plot, the link cable that was supposed to dump the asteroid into an alternate universe was explained using the teleporters in the game but also with the PC box.
The universe the meteor was exiting and the one it was entering were both very similar. There were differences, like no megas, but still very much alike. So where does ultra space fit in?
Anabel is a known faller, which means she's been to ultra space. We also know she's not originally from this universe. Given the other clues we have, we can say she's very likely from the non-megaverse, but that's unimportant. What is important is that in order to get to this universe from another, she had to go through ultraspace.
There's also the fact that if you transfer pokemon from the older generations to the new ones, you get the message "Seems to have traveled across both space and time to reach you in [location]"
There's thousands of Fallers out there, and the vast majority of them are sitting in your PC.