Are we sure this is the armor cap? I mean everywhere I look that's what it says.
But my Stalhrim armor has a rating of about 830. Well above the cap.

But when I switch to my steel set it's about 700, you'd think that it'd be the same because anything above 567 is ignored. But for some reason It seems like I get hurt faster in steel armor.

Is this all in my head because I'm expecting steel to be weaker, or am I really noticing a difference here?

it is all in your head, EXTENSIVE testing has been done, it is DEFINITELY the cap.

Just making sure

It is the cap. No questions.

Isn't there also a 100 hidden armor rating that's also taken into account?

567 is the cap because someone working for Bethesda likes numbers.

The cap doesn't keep an AR value over the cap from being displayed but a higher value doesn't do anything.Actually the cap is really 667 but there is a hidden 100 AR from wearing 4 pieces.so 567 is what you see when the cap.has been reached.

edit: for the numbers, 100 AR gives 12% damage reduction so 667 AR gives 80% reduction, a nice round number.

Visual rating of 567 (all four main armor slots occupied)* or 667 (no main slots occupied)* is definitely the armor cap. At that point physical damage resistance is 80%, the maximum achievable amount. Any additional armor above that is no use at all. If you think there's a difference above that point, it's all between your own ears.

* All armors occupying main slots (head, torso, hands, feet) have hidden rating of 25. This is a fixed amount for all armors using those slots.