Your data point noted, however I expect Cunard have done their maths, and looked at average daily onboard spend for single occupancy vs 2 or more in a cabin. If they take into account all onboard spending: bar, drinks packages, internet, shopping, casino, speciality dining, laundry, excursions etc a couple or a family in a stateroom will on average be spending significantly more than a single traveller, hence maybe their reluctance to double up on the OBC.