I see now.
I see why if you have "Armor II, Block II, Armor II" then it doesn't block everything. It only has about an 80% chance. One "Armor II" has a 40% chance, but adding Fortress Arms, with Block II, just multiplies that by 140%, giving you a 56% chance. If you add another Armor II (Diamond-Weave), it gives you 78.4%.