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

@[email protected] -> that’s me on beehaw, where I am writing from

@[email protected] -> that would be me on your instance but I don’t have a user over there

@[email protected] -> thats actually me on mastodon, which is another instance of another service in the fediverse.