The guy in the bottom left of the picture loses half of his head in one of the views. The half of the head that he loses is used to complete another guy. Since his head is not completed by a head from one of the other guys he becomes an extra guy. In other words, he is now only a partical guy making them 12 complete and one partical. I'm not sure if I have made any sence, but just pay attention to the bottom left guy.
Ryan