It is truly a mystery!! I have been upgraded twice on RCI. The first was my first cruise in '86. I booked a specific inside cabin during peak season, almost a year before the cruise. We were upgraded to an outside cabin on the same deck. In '00 I booked a guaranty cat.Q about 2 months before the cruise. This was in Dec and we were upgraded to an outside cabin.
My conclusion is; it all depends on what price cabin you booked, the time of the season and how early you booked and most of all did that elusive upgrade fairy visit you!!