Is it the description on the sword's stats that increases, or the damage listing in the inventory window? The sword itself should not change, AFAIK; there are several reasons the listed damage might.
Does the shield have an intelligence or magic damage bonus, etc?
The bonuses from the one handed expertise and sword and shield skills may be different (though that would effect normal and magical damage).
Are you duel wielding? Equipping the shield could have switched the averaged damage of two weapons (the off-hand with a large penalty until the dual wield skill is boosted) to the damage of one weapon. Again, though, that should effect normal damage as well.
