I am a bit confused by your line of thinking... As soon as a a single party cancels a reservation, by definition it will cause a day to not be "sold out." So if one of the parties who booked super early decides they would prefer a later departure, this is somehow a sign that the demand is "lower than expected?"
There is actually one departure available every 2 days (not every 3 days). That works out to 15 or sometimes 16 voyages per month. So approximately 9,200 rooms to sell. However it is apples-to-oranges to compare a Starcruiser voyage to a night in moderate hotel room. The Starcruiser is a super-premium, fully-themed experience and at the end of the day POR is.... just a hotel.