I was testing out how zombies "socialize" by spawning around 30-50 of them throughout a massive cave. They did what i expected, until I saw the zombies popping up randomly on the surface. I believe they were doing this due to an error in the coding, but I'm not sure. I have done tests on this and it keeps happening. The zombies keep appearing on the surface above the cave.
I've seen the new zombie spawning code spawn the zombies way up in the air (which isn't supposed to happen), so I guess you're seeing those who survived the mid-air plunge.