Normal zombie spawner is spawning zombies in blocks.

I have made a machine to purify zombie villagers to get villagers. I have several 1x1 dark rooms around the spawner where, in 1.7.10 zombies spawn normally. In the snapshot, the spawner is spawning zombies in the walls and floor of the room, and rarely in the rooms they were spawning in previously. In the included picture, there are zombies in the floor and walls of the room.