Um, Zool I have an item to disprove your statement, I have a set of Full Plate that weighs 40lbs and says it has 80% weight reduction so unless normal everyday Full Plate weighs 200 lbs the reduction is based on being X% of the original objects weight. So in my case the Armor is 80% the normal weight. That or I have bugged Armor.
Your armour is probably bugged, I've seen weight reduction not work before.