User blog comment:Lykrast/Runes: why I hate them/@comment-16737371-20140326150842/@comment-11045621-20140326170140

i think you confused pen with reduction: flat Mag-/ or Armor-reduction can bring your target under 0 resistance, but penetration can only ignore resistance till 0. Any additional penetration has no effect.

But you`re right with 10 AD by runes you have about 10% more damage on your AAs, and 15 AP at lvl 1 is really strong (depends on scaling).