Why are the Packers called World Champions?

I am all for them winning being a Wisconsinite, but why do they call themselves "World Champions"? I mean its the NFL, National (meaning this nation, not the world) Footbal League. So if it's just the nations' football teams, and the superbowl is held only in the US, then why are they touting themselves as "World Champions" when they are really just National Champions? Thanks!