It's a buffer.

It doesn't mean that the server is full, it just handles logins in batches as to help with managing to load from simultaneous login queues. If I recall this was a measure introduced to essentially prevent the logging servers from crashing when under strenuous load - For example, new patch, new expansion, and/or when popular content creators join the game.

It's not really designed for 'current' load I would say, but rather the potential for a significant load, where it tends to scale better.