You are probably more likely to be upgraded from a gty room just because the very fact that it is a gty means that they are "overselling" in that category. They know that a certain number of guests will have last minute cancellations, request to buy upgrades, etc. Also, if the ship is not filling as fast as they'd like, it is better to sell a 12 or 9 gty (put a family on the ship) than to say we are full in that category. Also, the lower category you have booked the greater chance for an upgrade just from a mathematical standpoint.
If you're booked in a 12 gty, there are 800+ rooms that you could be upgraded to. If you're booked in a 2, there are only 2 rooms that are possible upgrades.
However, we have been upgraded from an assigned room. There really doesn't seem to be any logic to the pixie dust.