The metal armor set bonus is for a different mitigation - with metal armor your max armor also provides mitigation. By default, your current armor provides the mitigation per 25 points. Or, in the case of that mod, every 20 points. But it's variable, since it's based on your current armor. So at 607 armor you should mitigate 24 points of damage per hit. But once your current armor falls to 307, for instance, you'd be down to 12 mitigation per hit.
We could/should add it to the damage mitigation page, although it would be a bit strange because it would change pretty rapidly during combat. The reason it's not there, is that that page lists attributes. The current-armor per-hit mitigation is not an attribute, it's more like combat logic... But one thing that is an attribute is the per-point value, i.e. 25 and 20. That could a lot more easily be displayed on that page