The issue is that Blood mastery scales differently to other tank masteries, so in old (soloing old stuff), nerfed (ie Heroic Firelands after the 30% nerf, or DS in the progressive nerf) or otherwise massively outgeared content, we scale better.
To add to this, DKs are the only tank with a mitigation mechanic that allows for player input to a large extent, in that if Blizzard expects us to be pushing 6 DS/min and we're pushing 12, we're gaining twice the benefit from Mastery that they intended. Also, pushing out twice the number of DSs tends to increase your damage by a massive amount.
To further add to this, a DPS stat (Haste) gives us a significant (tho, not as significant as "traditional" tank stats) survivability boost, by effecting the above figure.
So, between the massive buffs given to Blood in 4.3 (see one of my previous rants), and the above factors...you can use your own ability with playing the class well to mitigate the survivability loss from not wearing tank gear. It's no different to a shield tank capping hit/exp and stacking crit after CTC cap, except DKs have the added "benefit" of the (re)active mitigation model, whereas the other tanks do not.
When or if all tanks move to an active mitigation model, it's entirely possible that all tanks will work this way; instead of focusing on lowering damage incoming, they could instead focus on increasing their damage output. It's just at the moment DKs (and Druid, tho that's more based on them having access to synergistic Cat abilities) are best at utilising this method.