I thought there was just a damage reduction cap of 80%
They actually hard-limited AR to 600?
I think so. I remember discussing this months ago. People wondered why their rating stopped going up, even though they wore better stuff. For example, if you temper things fully to Legendary (That's the highest, right?) status, then you don't need to wear above, like, Elven or you're wasting weight because the extra armor cap provided by Ebony, Dwarven, etc isn't accounted for due to the hard-limit.
However they've recently altered smithing and other stuff. I'm not exactly sure anymore, but that's most likely what you're encountering.