You’re probably running into hardcoded spawns. Pools, parks, and apartment buildings (for some examples) will generally be populated at game start on Dynamic. Likewise, corpse “map_specials” have chances to spawn zombies and/or denizens as part of mapgen.
Dynamic will eventually be phased out. I know I loved having the grace period, then fighting more or less constantly when in towns, but I was in the minority there. :-/