I noticed today that I actually get one AC -less- when I equip my 4/4 chain shirt armour, than when I wear my 3/4 studded leather. I gather this has got something to do with the resent changes of bracer AC... Weird, though, still.

What's your DEX bonus?

+3, with 17 DEX, at the moment. Both armors offer a maximum DEX bonus of +4, so I doubt that is the issue.

Did you have AC bracers on, or have Mage Armor up?

Neither. It seems to have been a temporary thing, as it worked fine the next day, but I'm positive it didn't at the time when I reported it. I think it has got something to do with the way deflection AC is calculated, as I had recently gotten a spear with +1 defl. AC, and had, since earlier, two items that gave defl. AC versus specific things (specifically slashing, and aberrations). Now, why this would affect the AC added by armor AC is beyond me...