Mob Spawning

Mobs in a dungeon I found mobs where spawning at a light level of 12
i found a dungeon so I killed every thing that was in it then placed torches on the spawner so zombies wont spawn.

But instead a few zombies spawned. I checked if the light level was below 7 but it was 12. Making it impossible for mobs to spawn. This issue is only with spawners, mobs spawn normally in caves and on the ground.