I would guess you are booking a room for 4 people. We are traveling with a group needing 4 cabins. All of us got our rooms assigned at booking except for the group with 4 people listed in a cabin. The reason is that there are a limited number of cabins that will accomodate 4. So, they wait to see what is available and then divvy out the rooms. The longer the wait doesn't necessarily mean the better chance of an upgrade. What does mean a better chance is when you booked and how many other 4 person cabins are needed.
If you're booking less than 4 in a room, never mind what I just said.