afaik, there's several factors that will affect the damage, the type of projectile, the type of armor, angles, velocities, hit location, etc.
for example, there are some projectiles classed as penetrators, which aim to pierce a hole in armor and damage whatever's inside. these will usually create a clean entry hole in the armor
then there are some high explosive projectiles (such as HESH rounds) meant to make a large enough explosion such that the concussive force does the damage within the tank without penetration of armor
the location of the hit can also play a part, some areas are less armored than others, or, in the case of tanks, the stored ammunition can cause a secondary explosion, resulting in more damage
if you really want to get into details, the molecular structure of the metal in the armor may affect the damage as well, some metals or the manufacturing may induce brittleness or higher stress concentrations in certain parts making them more prone to breaking rather than bending under load
in short, theres really many variables that come into play and theres no one definitve answer, but i hope this helped give some insight ;D