A DCC increases the RC range, and the damage you receive is scaled by the factor (1.0 - your_distance / range), so suppose you are right at the border of the RC range and you get 0 dmg. Then someone builds a DCC, the RC range increases by 50% and voila, you get 1/3 of 40 dmg, which is 13 dmg.
I dunno if this is wanted or a bug, ask Norf for that.