Question for DBS Manga

So I'm reading chapter 26 of the Dragonball Super Manga.

Goku and Vegeta forced Zamasu to end his fusion and he started turning into multiple copies of himself. Before Goku and Vegeta attempts to escape back to their own timeline, Zamasu says something I find very odd:

"I have a time machine as well. The one that was made here in the 12th Universe!"

Now I remember the story we were told regarding the 12th universe and how a mortal managed to make a machine that could travel through time which led to the cosmics laws which says you can't do time travel.

However here is my question; Why is Future Trunk's timeline in Universe 12 and not Universe 7? I would have thought that the Future Trunks we know of is the same one from Dragonball Z and at that point there was no such thing as "different universes".

So what happened here? Why is Trunk's future *not* an alternate timeline of Universe 7? Far as I can tell the time rings are parallel worlds but it seems implied that they are parallel worlds *of the universe they were made in*. So when Gowasu and Zamasu goes to look at the barbarians in Universe 10, the alternate timeline ring is created must have been another universe 10 ring. But with that line from Zamasu about being in Universe 12 that now does not seem to be a given.

Is this explained anywhere?