There can be a couple of reasons why this is happening.
1. If I go to the RCI website and book a cabin, that cabin is now pulled from inventory.
It gives me a couple of days to make my down payment.
If I never make the down payment, that cabin is placed back into inventory.
2. TA's will hold cabin space for groups - all the time.
If deposits are not placed on the cabins within the allotted time, the cabins are returned
to RCI and end up back in the inventory and are available to book.
3. During the time of final payment you may see cabins come available that have been previously unavailable.
Passengers have up until the time of final payment to decide if they are indeed going to commit to the cruise.
If they decide not to cruise, the cabin is released to the inventory.
And this releasing of cabins (as in #3 above) is actually happening all the time, but it is seen more right at the time of final
payment, because it is [pay up or let it go] time!