It's a pretty clever way to handle the superhuman abilities. Instead of hunter = superhuman, you need the use alchemy to enhance yourself. Alchemy is performed using monster parts, gathered and sold by hunters. This enables the hunters to be further enhanced and gather better parts. Probably true for parts used for weapons and armor and magical crystal analogues as well. This also would explain why Alchemy Castle became respected and powerful. Kinda like witchers, come to think of it.