system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] Batman Beyond (Terry McGinnis) lives on Earth 12, which supposedly is the near future of Prime Earth. In Bendis’ Legion of Superheroes run, Terry is shown to exist in Prime Earth’s future. How is Earth 12 a separate universe if it’s literally just the future of Prime Earth?

Sorry, that title wasn’t very concise. What I’m wondering is how Earth 12 could be considered an entirely separate, alternate universe if it’s just Prime Earth in about 40 years. That’s just the future, not an alternate history or fundamentally different reality. Is it just that Earth 12’s universe is about 40 years older than Prime Earth, so their “clock” runs a few decades ahead of time? Even that doesn’t seem like a good justification for E12 being an alternate universe.
gpt
Universes are weird. Theoretically, you could have a universe that is exactly the same except George Washington faces right on the US quarter. So it makes sense that E12 could be running on the same happenings as Earth Prime but 40 decades earlier. Or it could be a nearly totally different universe that happens to have a similar Gotham and Terry McGinnis. Or anywhere between the two.