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 (link ):
"I believe the reason Riot reduced Armor Penetration on runes was because of the change in how the ArPen applies. Before, the "Flat" ArPen applied frist, and percentual came after. Meaning Flat ArPen was effectively reduced percentually by you percentual ArPen. This meant that having those "24.94" Flat Penetration together with your lategame (40%) and (10%), for a total of 46% ArPen:
( 1 - 0,4 ) - (( 1 - 0,4 ) 0,1 )
This really meant you had around 13,4 flat penetration, as 46% of 24.94 wasn't accounted for.
Armor Penetration marks was still stronger lategame, of course, no doub…
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