Ya, you can't put in just the EH calc for the piece of gear, there are a ton of different modifiers that go into it, an armor value that low using the M = A/(A+K) equation would make that small amount of armor almost worthless especially when compared to the small amount of HP on the item.
Basically 11armor ~ 1 stam. 800/11 ~ 72 stam vs 55 stam. Ony ring wins.

