Halivar said:"Harder to hurt," not "harder to hit." Joe's comatose body takes a beating better than Jim's. Do you narrate every missed attack roll as "swing and a miss?" Does full-plate make your players super dexterous all of a sudden? If not, then just say the level bonus is natural, accrued toughness (i.e. natural armor). It's John McClane armor.
But that's already built into hit points.