Why do tanks need Determination stat still when they have Tenacity?

[Snarky_Sunseeker]

This might be a popular misconception, because PLD got a shield and shiny armor, but it's actually the tank that has the least personnal damage mitigation. Both WAR and DRK outclass PLD on this aspect.

Now I'm curious. Time permitting, would you please explain the reasoning behind the above statement and what exactly is meant by "personal damage mitigation"?

Not trying to sound stupid; I'm just trying to see if we're thinking of the same thing.