The way it should work (I'm not super sure how it does work) is that the rules require the Keeper to know (and therefore the players to say) exactly how the monster is injured. The harm done, what the character is doing, specifically, and the kind of monster they're fighting all inform what moves the Keeper chooses for the monster's wounds.
You keep doing 2-harm with your handgun, and the Keeper can keep saying "You blow another hole through the Vampire, but it doesn't seem to care" But if you describe your character swinging for the neck with your 4-harm axe, and you roll 10+, the Keeper, making the world seem real, needs to describe something quite different.