Attack rating would be better looked at as "chance to do damage". The onager isn't terribly accurate, but if it hits, it has an insanely high chance of doing damage (which it should). Think of attack vs defense like this - someone swings a sword and hits you. Now, they may have hit your shield, or armour, and not done any damage. Or they may have actually wounded you. I'm not sure of the actual mechanics, but it's very likely they do something like a random number + attack vs random number + defense. If the first is higher, it does damage, if the second is higher, it does not.

Bh