Ugh, this has been said and said again on many a forum, yet still no one can be bothered to notice/research for themselves. Call me a whiner, but when it's in a lot of threads on this class discussion forum, it shouldnt have to be asked again.
This:
ArP is capped for 100% armour penetration at 1400 ArP on the character sheet, 100% effective ArP. This is a buff to YOUR OWN SHOTS. Armour debuffs on the bosses, such as sunders and Fearie Fire, lower the bosses armour, your armour penetration then 'ignores' the same % of the armour left after the reduction.
Lets give a bit of math to give an example:
lets say, for ease of sake, that a boss has 10k armour.
less 20% for sunders (I will only use sunders for this, but it all still applies, and you get the gist)
10k-20% = 8k armour.
This means that your ArP will negate a % of that 8k armour.
If there are no debuffs on the boss, and it has 10k armour, then your ArP will ignore the same % as before, but as the original armour number is higher, the % ignored, will also be higher.
75% ArP on a 10k armour boss = 2.5k armour.
10k armour with 20% sunders (8k armour) with 75% ArP = 2k armour.
And as the Poster above me said, you shouldnt be gemming ArP until around 900 static ArP. Until then, Agility nets you better returns
.
Bawk.
Note: The debuffs/ArP work in this way as otherwise there would be absolutely no point in getting 100% ArP if sunders gave you -20%, you would only need 80% ArP to be capped....WHICH IS NOT THE CASE IN GAME. And would make ArP capping way to easy and a lot less unique to end game raiding.