My leading theory is that both lemmy.world and lemmy.ml were in a list of 5+ “recommended” communities, and “world” is the only recognizable word that implies all-inclusivity. And now that the world population is so high, more people will assume that is the “default/correct” community.
I joined world because I figured it was a global community, and did not want to limit myself before I even knew what I was joining into. I may end up making 2 or 3 accounts just to have access to separate, possibly defederated, communities.