Penetrating Blow

here's a potentially stupid question

i always took "this attack has 20% armor penetration" to mean just that, meaning other bonuses to armor penetration do not apply.
am i wrong in thinking that? if strenght and misc. bonuses stacked wouldn't it say "has +20% armor penetration"?

has anyone tested it to find out?

First, that's not a stupid question. I think more or less you understand Armor Penetration.

Anyways, here goes: there are two types of AP, base AP and +AP. A skill like Penetrating Shot is simply base AP, so if you use something like a Horn Bow that has inherent 10% AP and Penetrating Shot with it, you'll only be getting that 20% AP. Strength is another base AP, so Penetrating Blow would over-ride it. Something like Judge's Insight gives +20% AP, so that would stack with your Penetrating Blow/Shot to equate to a total of 40% AP.

Hope this helps.