It is mitigation that helps them get from light armor (which they wear) level mitigation up to about equivalent to of heavy armor (which the other tanks wear). They do not mitigate over all any more than the other tanks, it is just a different way and apples to oranges.
Right and thankyou for answering a question that wasn't asked, which has nothing to do with the fact that they don't need to have the 50% heal proc from attacks whilst in tank stance. Either all tanks should have that, or none. And then there's still the matter of their high dps.