My understanding is that PVP has a flat percentage nerf before armour etc is applied as well. I've heard 60% but honestly, who knows. 65% is the cap for armour reduction, but other skills/talents/effects get applied after that - so can obviously surpass a 65% damage reduction.
Cant say for pvp but the guide I read stated that armour was for bullet damage only (i.e. shotgunners) other resistances are for grenades/skills shock/disorient/bleed/burn etc.
Got my armour capped at 4.7k (65%) tonight with a couple of decent crafts, will have to run some RC tomorrow to see what difference it makes.