One value determines where you get hit, the other how hard.
The percentage to absorb damage is the chance that a particular part of the body will be hit. The values for the boots, leggings, gloves, breastplate, helm and shield add up to 100 (presumably if you do not have a shield equipped the rest of the percentages go up, or at least for the head and chest).
The armour class of the part of the body that is hit is used to determine how much of the damage from a particular attack is absorbed. The total armour class displayed with the stats is just for general information, and not used in the actual combat calculations.