HenitoKisou: If enemy is not prepared or didn't shoot first weapons do more damage. If they are in agressive stance they are more immune to attacks and weapons tend to be less effective. From what I recall there is no defensive behaviour where they would be more prepared or protected, only passive and agressive (but if it's other monster, not player their aggro doesn't count toward less damage to them, AI is most likely focused on the player, so they can survive in some cases unless cutscene is triggered or player is in the area). Still, this can lead to broken stuff as this game doesn't loop scripts, preserve attack stances, pain chance or objects on the ground/in air. Most notable are flying monsters, explosives, bosses resetting health or weapons like voodoo dolls hurting the player upon load all the time.

Believe or not, since some of these issues were fixed, although quirks of the AI like above are still present in newer games like F.E.A.R (and mines/grenades can still turn into not available to trigger/pickup-able sometimes). This might be something with Monolith developing around the same engine but revising into other forks and generations. ;P