A Hot Fix was implemented internally in World of Warcraft patch 1.6.0. which affects resistances and many raid bosses got an increase in Hit points/health—according to Tyren:
Monsters 3+ levels higher than the attacking player were resisting spells at a greater rate than intended. This has been addressed, and players will now be able to land spells with greater success.
Boss monsters in Molten Core, Onyxia, Azuregos, and Kazzak had their HP increased to compensate for the increase in caster DPS they will see. The reduction in resistances outpaces the HP increase. Overall, this is a positive change which will increase casters’ effectiveness against higher level monsters.
Since this change has had a significant detrimental impact on players, we have hotfixed the bug so that players can use their spells and abilities in these situations to their intended effect now rather than waiting until the next scheduled patch.