Originally Posted by
Lumi
I believe that an area will only allow a certain number of creatures to gen within its borders at any given time.
When you kill a creature, there's room for one more. The area randomly generates a creature of some type that exists in that area. If it happens to be a lower level creature, the balance becomes a little skewed. If this happens repeatedly, you eventually end up with a high critter population within that area, but they're all low level, being ignored by the scripters, and thus preventing higher level creatures from spawning there.
It's not really about the level, it's just the randomization of the spawning algorithms. If it were an area with three different creatures of the same level, and you were scripting there but only killing one type, that type would eventually be driven out, too.