skeletons, creepers and zombies spawning in swamp hut

Made a witch farm at a swamp hut. No witches spawned. Instead there were skeletons, creepers and zombies. The area was enclosed and no mobs were able to get inside of the area from the outside.

Seed: 6252938272653771370

Location: X=1140, Y=72, Z=-508

Difficulty: Hard