Could be % based, could be active player accounts on the server within a given month, could be a manual toggle that they just won't turn off despite it no longer being the case, who knows.
The login queue however is entirely artificial, because the login server works in batches.
This doesn't mean the server is actually full when you get a queue at 4am, but that X amount of other players are currently trying to log in and the server waits an arbitrary amount of time until it processes them all. Probably requires less resources than processing every login request individually.