Gannett Swan

Apologies if this wasn't actually a bug. I just assumed that the enchantment was meant to operate exactly the same in all cases. Anyway, thanks for helping to resolve it.

I just tested it, letting two skeletons fire at me 200 times, with two pieces of armor enchanted with unbreaking 3, and two without. The two unenchanted items(boots and legs) took the correct 200 durability damage each, while the enchanted items took 124 damage(helm) and 137 damage(chest).

Note: This testing took place in both a SMP server and my creative world(while in survival mode, obviously). It's the same everywhere.

Unbreaking creates a chance of the tool not losing durability when used/hit. Each level of unbreaking decreases the chances of it being used by half; unbreaking 1 doubles its lifespan, unbreaking 2 triples it, and unbreaking 3 quadruples it.

That information does seem to be outdated. Here is a screenshot showing how I determine my item's remaining durability:
http://i.imgur.com/cvQKr.png

I just tested it, by sitting and letting a skeleton shoot at me 20 times. I have the minecraft durability counter enabled, which is what I use to test this whole deal. Here are the stats:

Chest Starting Dur.:236 Final Dur.: 223 Dur. Lost: 13

Legs Starting Dur.:186 Final Dur.: 166 Lost: 20

I counted each shot, totaling 20, so I don't know where your idea that each shot reduces the durability by two comes from.

Apologies, I'm new at this. I assume I was hit by 63 separate attacks, since the legs had no unbreaking enchantment to reduce that number, but as you said, it's difficult to say precisely.