It's not a bug, I think its about weapon speed. I tested it right now on my 57 hunter. I used the Steelarrow xbow versus a Siege Bow which is level 53 with 48-90 damage and 2.8 weapon speed and with Steelarrow I had 292-309 damage whereas with Siege Bow 263-307 damage. I even remember that when I upgraded Steelarrow with Verdant Keeper's Aim the low damage was higher! Ofc it has less DPS due to weapon speed but but in terms of Burst damage and high aimed shot I am 100% positive that no other weapon beats it on this level range.