Why are minions taken damage increase as much as 25% from turret's AD?

For example, In Summoner's Rift, When outer turret's AD are 164, melee/caster minions are taken 205 damage and siege minions are taken 143 (70% of 205) damage. Although minions AR are 0 and turret's AR pen is 30% by Penetrating Bullets.... How is it calculated? Are there some implicit passive?