User blog comment:Nhan-Fiction/(Champion Remake) Fiora, the Grand Duelist/@comment-4881935-20131229192346/@comment-24087642-20140103171415

Actually, armor penetration works better against enemies without much armor.

Calculation for armor penetration: reduces enemies armor by how much armor penetration you have

Calculation for armor: reduces physical damage by an exponentially decaying amount

So, if you use armor penetration on an enemy that has little armor, you are increasing you're actually shredding armor that matters a lot more to them than it would tanks. The only reason that this usually is ignored is because most items that have armor penetration also have a passive that ignores a % of the enemy armor. So when you're giving her q armor penetration, it's actually making her stronger against enemies who don't stack armor.