I'm guessing it's something like this:

Say there are 20000 spots available on the server. If there are more than 20000 people, then you get the 'try again later, server is full' message. If there's 18000-19999 on, then the queue opens up and starts letting people in. If there are 19999 people on, and the server opens the queue and tries to let 500 people in, then 499 of them are going to be pissed off because the person at the front of the queue got in when the system fucked up and added more people to the queue than there were spots available.