There's Armor Class, which is displayed on your Inventory screen, and then there's your Armor Class Modifiers, which are only displayed waaaay down on your Record screen (at least they are in BG2, it's been quite a while since I played BG1).

Certain armors will protect against certain attacks better than others. Chain Mail, for example, works great at blunting the effects of swords and arrows, but is no good whatsoever at blocking attacks that are already blunt, such as maces. Therefore, Chain Mail has a +2 Armor Class Modifier against Blunt weapons. So you take the armor's base Armor Class (5, I think), add in the AC Modifier, and you have an actual AC of 7 against Blunt weapons, which is 3 AC points better than just going naked. Now, Full Plate Armor works great at deflecting piercing, slashing, & missile weapons, and you will see those AC Modifiers on your Record sheet when you wear Full Plate.