Protection items need proper direction

Sunfire Cape 500 health , 50 armor Deam Man's Plate 500 health , 50 armor Randuin's Omen 500 health , 60 armor

Sunfire Cape Deam Man's Plate are offensive items for tanks and suppose to give less protection and more offesive power

Randuin's Omen is Defensive is more focused on protection and therefor should give extra protection stats

for instnace Sunfire Cape 400 health , 40 armor and burns nearby enemies for something like 5 + 1.5% max health in magic damage so Randuin's Omen should give 600 health and 60 armor because its more focused on protection and not on damage

In other words you chose if you want to be Offensive Tank or Defensive Tank Defensive Tank means you get more protection and less damage Offensive Tank means you are focused on damage and less on protection

Thornmail is actually suppose to counter Critical Strikes because the more damage you receive from enemy basic attacks the more damage you inflict to your attackers yet Randuin Omen gets Critical Damage reduction where suppose to get crit damage reduction cause its suppose to counter crits