Upgrades are a bit of tricky topic.
People book the cheapest inside cabin, and are told they got an upgrade when they move up two decks to an inside cabin (when in fact I'd consider these pretty similar cabins). And though you're up a couple of decks you may have been moved from mid ship to the bow of the ship.
I'd like to hear about incidents where people felt they got "real" upgrades... and why do you think it happened? Was it because you're a long time repeater with the cruise line? a first time cruiser with the line? you booked early? ... or did your TA tell you "they got you an upgrade"?