Monsters walk off cliffs of high places that would make them take a lot of damage without any water, trapdoors, just like it was normal.

So I was just branch mining, where I mined into an underground ravine, and all around me, creepers and skeletons were just walking off about 8 to 10 high blocks cliffs, falling that height just to come at me, but aren't they supposed to stay on block that high since they should know they would take a lot of fall damage? There wasn't even any water and obviously not trapdoors, this was a new ravine. I'm kind of confused. Can you help?