This is not a dirty little HAL secret. The two other cruise lines that I have sailed do the same thing. When shopping for cruises for two passengers during early booking, they only show the cabins for two. As the lower occupancy cabins sell, they open up the higher occupancy cabins to all. I test this by making dummy bookings for three or four. Voila! Those cabins with three or four births open up. I call my TA and tell him which cabin I want and have never been denied.
Why they do this? The cabin price is the same whether there are one, two, three, or four passengers. But the bars, casinos, gift shops, specialty restaurants, and excursions make more money on the higher occupancy cabins. Also, they can’t put four passengers in a cabin for two.
I wonder if the Upgrade Fairy might make an offer to you if she really wanted your cabin for someone else.