This is a subject that is always around on these boards.....
DCL does upgrade, but it is a random process and no one has figured out how and why DCL does it. You will find many on these boards who have been upgraded (me included....13 DCL cruises....5 upgrades) but overall it is a small percentage of actual overall cruisers.
You could have booked early, booked late
Had a GTY or an assigned cabin
Been in a low category, been in a higher category
Paid off cruise early, paid off cruise at due date
You could a first time cruiser or a repeater.....etc...
There is no rhyme or real reason other than possibly moving folks from the less expensive categories to the higher cost categories to make room for more people to book at lower cost (the cat 10's, 11's & 12's seem to get upgraded the most).