[LiA] How is backstab damage calculated?

[Vio]

I just tested backstabbing a different enemy (Adult giant spider) of 59 Health, 30 Armor, and Slsh and Acid Resistance, and that enemy can be killed with one hit (60 Backstab damage). Now I'm confused.

I suppose it's because a spider has a head, while an evil eye only has a body, if I'm not mistaken. Head hits are more powerful.