What is this?! Spawner bug in version 13w09b!

When I get close to a Spawner what i made, he start to spawn under ground a lot of of mobs. But, the real mob is in another side, when the real mob die, another spawn, the spawner have the propertie of Entitie Cap = 1. But under ground spawns more, and when i go to the fakes mobs, i can hit they, but when i log out and enter again there are not more there. But, again, if i get close to the spawner, they start appear again. Video soon: http://www.youtube.com/watch?v=0FLgno47Q-w&feature=youtu.be