I think there may be a line of sight element as well.

You can see them pop in but are typically already spawned. If you are in an area and kill them off new spawns will tend to be behind things from your line of sight.

So if I want to keep an area clear of random spawns I find it easier with removing the clumps of trees and ground clutter.

I think this may give the impression of grass spawning mobs... I have kept my foliage off for a long time but I don't know if that level of stuff would actually work that way or not.