Shield mitigation

I really dont understand the damage mitigation on the shield. When i face the tanky guys in the Crucible, when i hit them and they block with the shield the damege they take is almost 0, however when i block an attack by some mobs they can even take off 30/40% of like even if blocked. I really am having difficulty in grasping the mechanic.
Does this mean that the guy with he shield in the Crucible is probably over 1k of armor and that is the reason for so much damage mitigation. Anyone that has played with over 1k armor has experience the same damage mitigation. Bye bye and thx for the feedback