I was thinking the game might give a blanket +armour bonus for back hits, but seems not. Its probably code from when back hits from arrows got stuck in shields like they are deployed?
Anyway i'm currently testing out Plate Covered Round Shield. If I read this right its 61 + 10 so 71 extra back armour if something hits the shield. So with my plate 144 armour in that area. Still nothing beats eliminating damage entirely so don't think its a substitute for deploying it, just in a melee fight it might be alright, though it weighs nearly 8