in bc, some raid bosses had lower armor than others, giving melee an advantage or disadvantage over casters in dps depending on the armor. in wotlk, testing has shown that every raid boss has the exact same amount of armor, which is 10,643.
part of this i think is due to the way armor pen is now. going from a flat armor reduction in bc to a rating system now. having a flat boss armor value will ensure that arp will always be the same from boss to boss.