This is strange umbarians scavenge the gondorian corpses for wearing their armors or wielding their equipments. Mevans removed a long ago the ability of NPCs to pick up items at the floor if they are better then theirs (because it was OP or a sort of thing). It sounds like a glitch for me.
I never experimented this on my own game but I never tried to raid Gondor with an umbarian armee. I should check it out.