It’s mentioned on the wiki how through-armor HP damage is calculated. Which is to say, 10% of armor is subtracted from through-armor hp damage. Head hits give a bonus +50% damage against hp (assuming any gets through).
Needless to say, Orc Warriors have 400 body armor. Probably somewhere around 200-300 head armor if they’re wearing any helmets. So they’re shrugging off woodcutter axes like it’s nothing.