Not sure if Disney has a rhyme or reason that we know of. One thing they have to balance is the life boat capacity. While there may be staterooms available, there may not necessarily be room in the assigned lifeboats. (I mean there is room, but they don't over crowd them because they know that sometimes you just end up in the first available). They may also consider workload for the stateroom hosts. Caring for full rooms of 4 guests is probably more work than the same number of rooms with only 2 guests each. Finally, there are also rooms across the ship they hold back for a number of reasons, scheduled maintenance, deep cleaning, and guest recovery rooms (like if your room is leaking water on the carpet or the bathtub overflows) and they need to put you somewhere else.
They may look at rearranging or any other factor. They could be holding it back to see if you get one of the corner rooms, which would be a handicap style room as well. So they need to see if someone else would need it first.
