User blog comment:Emptylord/Blogs/Reworked Runes (2015)/@comment-24795478-20150515125212/@comment-4022742-20150518081906

The current "retard" number is something that came about by rigorous testing in combination with masteries and base/growth stats. Increasing or decreasing the rune value will mean a rework for all other stats too, in order to keep the balance.

An increase in AD/AP will increase item and skill damage, while an increase in raw armor doesn't have that scaling advantage. So to keep the power creep in check both the base armor and growth armor has to go up for ALL champions....