You need to adjust your thinking a tad. The instances are all separate servers / websites that have agreed to talk in a certain way, that happen to use identical software. So it's like going to Amazon and creating an account and then going to Imgur and creating an account. No one cares that there are two identical usernames as it's two entirely separate databases.
Where it gets more relevant to Lemmy is that your username has @instance.abc after it.
Reddit was so American too, all the arguments and things seemed to be through their world view. The fediverse should allow much more diversity, and be a bit more multicultural