In the first screenshot, you'll see that it appears to give 12,000 (green text) damage, but has a negative sign on the number, instead.
In the second screenshot, you'll see that after moving the bow to a different spot in the stash and back again, it changes to 10,000 damage, and again has a negative symbol despite the green text.
Screenshot 1: http://imgur.com/Pqw2D
Screenshot 2: http://imgur.com/Dk1ZD
In either case, I wouldn't expect more than approximately +400 damage.
Anyone else notice this before?