NPCs with combat level 3 or less should be hard coded to do zero damage
Replay evidence shows 5000 trials of fighting chicken with full iron and not a single hit. The expected hit rate is 4/256, or 1.5%. We should thoroughly expect to see a hit with 5000 trials.
Another alternative is that there was a mismatch between client and server NPC stats. If so, then chickens and level 2 rats had 0 attack to achieve the same result.