The A300-B4 has a minimum flight weight of 191800 lbs. A typical empty weight (mass) for a freighter is around 181000 lbs so if we are empty we need about 11000 lbs of fuel as ballast.
The only reason anyone has given me is that the flight test program did not cover these very light weights since as a passenger A/C it would never be below 191800 lbs. (seats, galleys, etc.)
I don't know if Boeings have the same situation.