Bosses who appear as Skull have a resistance, dodge, parry, etc etc that is calculated as if they were 3 levels above the player.  If you're level 70 and fighting a "skull" level boss you have the same chance to miss, get parried, spells resisted etc as if you were fighting a level 73 mob.

BUT their HP, damage per attack, etc is set independent of that - they're not normal mobs.  So if Blizzard wants some boss to have ten million HP and hit for 20k per hit they simply make it so.  Normal mob level references don't apply when you see a "skull".