No, its because its additive mitigation (compared to something like Barkskin or Survival Instincts, or Shield of the Righteous). Ex: 0% dodge, SR is 45% mitigation. At 20% dodge, SD raises your dodge to 65% which is the difference between taking 80% of the damage or 35% of the damage, a 56.25% reduction.
The problem is that your dodge doesn't really increase that much with gear so the scaling of SD's benefit is pretty negligible compared to something like the scaling of Shield of the Righteous for prot paladin. Since Blizzard cares about the effect of secondary's far more than anything else, mastery has to be impactful or the stat would be worthless. This is part of the reason why it is diversified into also granting block chance so the increasing returns on SotR's damage reduction doesn't get too out of hand.