The "lemmy.world" instance is hosted by a user called ruud so you could ask him directly about the naming choice. However it looks like lemmy.world is one of the instances that is hosted on a quite powerful server so you could see it as the instance that is capable of hosting for a "large part of the world" meaning a lot of users and a lot of communities compared to other instances. Edit: If you don't really like this instance you can of course sign up on other insances and try them.