The bonus in question is the "extra" armor on items. Take the old PVP items for instance where the armor value is in green tekst because it has more armor than a usual item. An item like that would get the 370% increase calculated from the base armor and then the "extra" armor as a flat number added on top.
Ekample:
Vindicators leather belt
Armor 258
In bear 258*4,7=1212,6
Vindicators dragonhide belt
Armor 300
In bear (258*4,7)+42=1254,6
The 258 armor in the last example is a best guess based on other items on the same level. Also i didn't use all the extra armor from talents and whatnot, so please don't knitpick the math. It's just as an eksample of how litte we get from bonus armor after the patch.