I was in a mineshafter when I've seen lots of cobwebs together. It means a cave spider spawner! But when I've destroyed all the cobwebs, there was no spawner! I've generated the world again and it is true! that dungeon doesn't have any spawner!
SEED: 70805752
X: 309
Y: 12
Z: 89
This a "feature" of the random world generator.