Armor penetration or ArP is renowned for being the most confusing, poorly worded, explained, and implemented stat in World of Warcraft. It is partially the reason why Blizzard decided to remove it from the game starting in Cataclysm. Thankfully, on Molten-WoW ArP is now working nearly 100% as intended, and so our lives as hunters are easier.
The reason Armor Penetration is so confusing is the way it’s worded and the simplistic way all humans like thinking. Everyone can assume that 1% Armor Penetration (14 ArP Rating) will give you a 1% damage increase. This is wrong. I realize that it seems like it would be a kind of thing you would say “no you’re wrong! 1% less Armor means 1% moar dps, but let me explain. The reason there is so much confusion as to what ArP does, and how it affects our damage comes from an incorrect understanding of Armor itself.