Because as you have previously mentioned, penetration and injury are different things. If I shoot you with a ridiculously high powered 3mm bullet that has the same armor penetration as a 5.56mm round and as 12g shotgun slug, they're all going to penetrate the same distance, more or less, into your body. But the 5.56mm round is going to destroy around 4 times as much tissue, with a corresponding increase in the chance of hitting something vital, and the 12g slug is pulverizing around 30 times as much material. So the 3mm bullet does 5d pi- for an average of 17 damage and 8 injury on an unarmored human, the 5.56mm bullet does 5d pi for an average of 17 damage and 17 injury, and the 12g slug does 4d+2 pi++ for an average of 16 damage and 32 injury.
Similarly, if I hit you with the blunt end of an axe's head, I'm doing crushing damage and I might break your arm. If I reverse the axe and hit you with the sharpened edge, I'm doing cutting damage and severing skin, muscles, and tendons as well as possibly cutting through the bone. It's the same amount of energy, momentum, whatever in both cases but the chopping head creates more grievous wounds.
It's not hard to remember the wound modifiers if you play for a bit, or you could just write a tiny lookup table somewhere:
Code:
bu x1
co x1
cr x1
cu x1.5
im x2
pi- /2
pi x1
pi+ x1.5
pi++ x2
tx x1
But it's your gaming table, do what you want.