Talk:Stacking/@comment-4490419-20120104235338/@comment-4087140-20120105001356

You must interpret "armor penetration" as "effective armor multipliers". 40% armor penetration = armor is multiplied by 0.6; 10% armor penetration = armor is multiplied by 0.9. As a result, 0.6 × 0.9 = 0.54, and a multiplier of 0.54 is equivalent to armor penetration of 46%. See the section "Stacks multiplicatively" in this article.