20 armor decrease 1 damage from the max damage after that the min and max damage are compared, whichever makes more damage is taken into account. You have say 100 armor and the enemy has attack 5-9, 100/20=5, hence 9-5=4, so the minimum damage is taken into account since it is 5 and is more than the 4. Critical hits inflict x3 the max damage, so large armor is essential to survive those...
Weapon proficiencies and the later mastery skills increase both min and max with the particular weapon.