My Barbarian has about 46 dps using two weapons that are both about 15dps. When I equipped a shield, the damage went down to the low 20's, which is exactly what I expected.

However, when I unequipped the shield, and then hover over it, the tool tip now shows that the reduction in damage is only about -5, even though I am replacing a weapon that does about 15dps. When I equip it, my profile shows that I do about 41 damage now.

So, in summary, the display seemed to work correctly the first time I equipped the shield, but after I unequipped it, it was broken.

I took two screen shots that show the broken damage display: http://imgur.com/a/GEuLH