Talk:List of runes (Historical)/@comment-5041438-20120725182758/@comment-4087140-20120726073658

Armor penetration applies to all physical damage, which is likely the type of damage being dealt by abilities that scale with attack damage. In addition, AD scaling is at the whim of the ability (one ability might have a 1.0 ratio, but another could be 0.35), but armor penetration always applies its full effect regardless of whatever scaling the ability has.