These are my personal observations based on my own experience...may or may not be accurate. We have been upgraded from gty rather than from a chosen room 2 of 3 times...despite many more bookings where we selected rooms...so gty seems better.
Off season seems better..ship is generally less full.
Lower categories are upgraded more than higher.
More likely to be upgraded if you are a single family booking rather than adjoining rooms or a family in 2 or more rooms.
Dumb luck?