If you're watching on xfire as a map ends, there will be 12/12 players, but as the new map starts it will show 0/12 and if you keep refreshing it will build quickly back up to 12/12.
If someone clicks join sometime when it is less than 12/12 they have a chance of bumping out one of the 12 players that was already there.
If they do this intentionally, then grrrrr is all I can say.
The other night, omeone quit as the map ended, so there were 11/12 players, and two people saw that and decided to join, but did so as the map was restarting. They both got in and I got bumped! That was pretty unlucky, but it can happen innocently.
So if you're wanting to join a server, the most considerate thing is to wait for it to restart, and look to see if the server still shows 11/12 and then click join.
Also, when people join on map change like this, halo almost always puts them on the wrong team and the teams end up 7 v 5. So if you do join near a map change, you ought to check the teams and switch if you've made them uneven.