Armor Piercing Ronuds does less damage with reduced bullet resist

[Gidzy]

Dummy armor = 61%



Bullet damage =136



Without proc armor piercing 136*(1-0.61)=53,04



With armor piercing proc 136



Without armor piercing proc but with crippling shot 136*(1-(0,61-0,18))=77,52



With armor piercing proc but with crippling shot 136*(1-0,18)=111,52 ?????
?WHY?