I am not sure that your assumption is correct.
80% full could refer to numbers (in fact the cruise to which you refer was just under 82% full) rather than cabins occupied.
For some officers, the number of guests is, I would argue, more important than the number of cabins available.
But, I suppose, either interpretation is equally valid.
On our latest cruise (an expensive 21-day repositioning), the ship (numbers and cabins) was nearly full.
On our previous cruise, there were far more solo travellers. Now, if the cabins were all occupied, it could be said that the ship was full. But, as one moved around the ship, went to restaurants, events, etc. it was obvious that the ship was not full.