because each instance will append it’s address to the end of your username, it technically is a different username per instance, for example:

@brunofin@beehaw.org -> that’s me on beehaw, where I am writing from

@brunofin@lemmy.ca -> that would be me on your instance but I don’t have a user over there

@brunofin@mastodon.social -> thats actually me on mastodon, which is another instance of another service in the fediverse.