Warriors sunder acts as a debuff to the boss armor, ARP ignores a percentage of whats left.
A lvl 83 boss has 13083 armor, the armor constant for this boss, C, is 16635 (i have included ghostcrawlers formula at the end).
To figure out how much of our ARP is beeing used without any debuffs to the boss armor, you take (boss armor + C)/3 = 9906, and multiply this by your ARP value, say 90%, giving 9906*90%=8915 armor reduced.
The remaining armor of the boss is (13083-8915)=4168 armor, meaning the attack goes through against 4168 armor.
Now, reduce the boss armor by 25% (FF and sunder).
The armor constant, C, is still the same, but the boss armor is now 13083-25%=9812!
(boss armor+c)/3 is now 8815, multiplied by 90% gives 7934 armor reduced.
The remaining armor of the boss is (9812-7934)=1878 armor, meaning the attack goes through against 1878 armor.
Since armor scales, your warriors should really get sundering
they are missing out on aprox. 2300 armor! You can calculate the dps loss here yourself, but thats a quick explination to how the ARP works. The thing is, the tooltip can be a bit misleading to the young mind
The code behind ARP explained by Ghostcrawler is as follows;
Computing the cap is a little tricky unless you are already familiar with how World of Warcraft armor works. There is an armor constant we’ll call C. C is derived as follows (in some pseudocode):
If (level<60)
C=400+85*targetlevel
Else
C=400+85*targetlevel+4.5*85*(targetlevel-59);
For a level 80 target, C=15232.5. For a level 83, C=16635.
The cap for Armor Penetration then is: (armor + C)/3.
A level 80 warrior creature has 9729 armor. C=15232.5. So, the cap is (9729+15232.5)/3=8320.5. Let’s say a player has 30% armor penetration from armor penetration rating and no other modifiers that complicate the calculation (talents, Battle Stance, Sunder Armor, etc.). The game chooses the minimum of 8320.5 and 9729, so 8320.5. That is multiplied by 30% = 2496.15, and so that much armor is ignored. The effective armor on the target is 7232.85 (9729-2496.15). From a player point of view, the armor penetration rating didn’t ignore the full 30%, but instead ignored 25.66%. (85.5% as effective as expected).
The tooltip is not actually inaccurate, as it states: “Enemy armor reduced by up to 30.00%.” That "up to" is key.