This is my take on the recent change in LoL's Armor and Magic Penetration system. I recently posted the following comment on YurdleTheTurtle's discussion:
I believe the reason Riot reduced Armor Penetration on runes was because of the change in how ArPen applies. Before, the "Flat" ArPen applied first, and percent came after. Meaning Flat ArPen was effectively reduced percentagelly by your percent ArPen. This meant having those "" Flat Penetration together with your late game (40%) and (10%) for a total |1 - [ (1 - 0.4) x (1 - 0.1) ]}} (46%) ArPen. This really meant you had around flat penetration, as 46% of wasn't accounted for.
Armor Penetration marks were still stronger late game, of course, no doubt. But in Season 3, the Armor Penetr…Read more >