This bothered me the first time I saw it, but is there a reason that Pierce Through is "You ignore 25% of your enemies' Armor." and not "Add half your Strength to Armor Pen and your penetration can bring enemies' Armor below 0 for the purposes of damage calculation."?
I assume it has something to do with the differing levels of Armor, Ward, and Focus, and making it so each Perk grants a similar buff in damage, but I can't tell if the difference is a buff because enemy Armor is too much higher than Ward and Focus or a nerf where letting Armor go below 0 causes a lot of trouble for something.