The room only fit 7 people. I'm assuming the 8th Cage was this one here, with a weird sign that was empty on both sides (probably to mark that it can't/shouldn't be used for this round) Was a pretty neat thing I found, wonder if it meant anything
That sign has to do with the plugin recognizing where a player will spawn. Chances are there was some kind of malfunction and the sign wasn't removed/replaced with a player for that round.