This problem had been explained over a year ago by Ghostcrawler (The Great Item Squish (or Not) of Pandaria). The part which still bothers me the most is what he mentioned in the paragraph "Mega Damage", where he stated "Even today, tanks can hit the ten digit threat cap on some encounters.". Several times i've now seen really wierd things on Gara'jal, when there was no chance for our tanks to get the boss back from a dd and Gara'jal just killed the dd.