Although PLD have lower dmg while tanking, their dps is sightly higher compared to warrior while offtanking considering PLD are more defensive tanks while warriors are more aggressive and high dmg tanks. It's obvious to know why they have lower damage.
Warriors need to do quite a few things to get more mitigation. Keeping Storm path up, timing IB for big attacks. While PLD mitigations are just a button away, their 90sec def cd rampart with compared to warriors foresight is just clear which is better.
Also even tho the self heal from warriors are low in high lvl raids it does help as they need the self heal to compare with the increasing block rates and strengths of PLD shield. If PLD gets a dmg buff then warriors need a mitigation buff or self healing buff