Since some powers can lower the amount of damage you can do, damage is listed to include the 100% "base damage" portion. If you hover over the damage line, it says:
"This effect alters the damage dealt by all your attacks.
As some powers can reduce your damage output, this bar has your base damage (100%) included."
Diellan has said in another thread that he'll probably remove the plus sign out front since it's confusing.
edit: I just thought. Maybe it could be switched to act like Hasten. Hasten says +0% in the display at 100%, and goes from there, allowing powers that reduce recharge speed to take it below zero, and powers that increase recharge speed to take it above zero.
Damage, on the other hand, is displaying +100% automatically. Maybe Damage could say +0% in the display when it is actually at 100% in the back-end to include base damage. That way the two buffs that function the same in that window, are displayed the same in that window.