idk. i've thought i'd noticed this behavior before, but never really investigated it. however, i just leveled the swamp in a huge swath around the bonemass altar without putting up any benches etc (outside the one's i have at my portal very close to the altar). before i did, the number of mobs in this area was nuts. now the only things i see once in a while are wraiths which are coming in from a distance, and 2 skeletons which are spawning where there used to be a bone pile. there are no more draugr, leeches, or abominations anywhere nearby.