Displayed AC is both avoidance and mitigation AC. That's why your AC goes up when you cast an AGI spell (and you're under the cap).
Avoidance AC from items is added to your displayed AC and, as Unmei says, raw AC from items - and tribute, which is just a hidden item - is subject to a multiplier. Buy a DoN earring, check your AC, plug in a 5 avoidance aug and watch your displayed AC go up by 5 if you want to check.
Magelo used to be great for working out what was avoidance and what was mitigation AC, but damned if I can find out how to get the figures now! It used to be by adding "&debug=true" onto the URL, but that no longer works.
FWIW the defense skill is the main source of avoidance AC, but even when naked you still have some mitigation AC. Fun!
(ATK is a similar composite between "chance to hit" and "power", FWIW. It's a shame we can't get at the raw numbers directly IMO.)
And on a purely theoretical basis, should I manage to find myself in front of a mob called "Test Seventy Five" I plan to do a
lot of melee-related parsing.