I *think* (meaning I read it somewhere) there's a hardcoded problem with armour - that it works binary. All or nothing. Either it deflects the strike completely or it lets it through completely. There might be some minor injuries that get through but they're just bruises, which means next to nothing. In other words: it seems impossible to win with a copper weapon against a steel armor. The better material always wins.
An example that happened to me yesterday: a dwarf of mine got ambushed and was promptly knocked unconcious. There was a human swordsman and five goblin archers fighting him. The archers emptied their quivers (50 arrows) each while the swordsman was stabbing the unconcious guy, then proceeded to bash him with the bows. After a looong time, they managed to sever his hands, ears and nose, while most other parts were uninjured. Reason? The dwarf wore iron armour, and the swordsman's copper sword wasn't able to do him any harm.
I know there's a bug where unconcious creatures don't die of bleeding, but that doesn't explain why didn't they stab him to death in the first place. Armour shouldn't work on a simple YES/NO basis. Call me heretic, but the old percentual system was much better.
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EDIT: More on topic. It seems it's not really a problem of wrong RAW values, much more a problem of the code itself.
EDIT2: Though he armour wouldn't be that much of a problem if (a) broken limbs and organs actually hindered fighting abilities, (b) creatures were able to finish off an unconscious opponent easily (like drive a dagger through his eye or something).