I guess I'll explain this. Your damage stat is the max damage you can do. So a stat of 15 means 1-15 damage. Armour is like additional health, but armour piercing is an amount of damage that comes before damage, but can only attack the armour stat, but never varies.
For example, an enemy has 16 armour and 10 health. You have 15 damage and 15 armour piercing. You roll a five. The 15 armour piercing is dealt to the armour stat, so the enemy is down to 1 armour, then the damage is dealt, destroying the armour and bringing the enemy down to 6 health. However, armour piercing damage is never dealt to the health stat.