The cruise lines have computer yield management programs that will tell them
exactly how people spend their money including who, when, where and how
much. If selling to single passengers was more profitable "overall" then
selling doubles, all cruise lines would be marketing to singles instead of
actively discouraging them from booking.


They may not have done the math comparing what singles spend compared to
what doubles spend. They may not have broken it down by other factors.
It's possible that a single booking cattegory X, on average, spends
more then doubles in cattegory Y.