To the OP:
Remember, there is a damage value assigned to each shell. Its value decreases with distance and armor penetrated (kenetic engergy carried by the round decreases with both). Heavier shells (should) maintain their damage potential better over distance/armor penetrated, as they rely less on velocity, and more on mass to get kenetic energy. A light, fast shell won't do as much damage as a slow, heavy shell, even if their respective penetration values are near equal (M8 at point blank range, M4 at 1000yds).
That being said, the system is flawed in my opinion. A 37mm shell will still set off the amunition and kill/wound crew members if it penetrates the turret. Infact, it would be better than a 75mm shell in some cases. Where a 75mm shell might tear though 1 side of a panzer IV's turret, and go through out the other (very destructive in its own right, but theres the potential to do no damage if it misses ammunition and crew members), where as a 37mm shell is more likely to just bounce around inside of the turret instead of passing through (greater chance of killing crew members and igniting amunition).