Agree that if you want a better room, better to reserve & pay for it. Think it's fairly rare for the upgrades...either something was drastically wrong with the original room, or they overbooked a category, or there is a pretty obvious marketing campaign going on.
When we were upgraded in January to the Club Level at the Contemporary, I think it was very obvious why. First, we'd already paid for an upgrade from the garden level to Magic Kingdom tower view, so the marketing department already knew we had some discretionary $$...so giving us a "taste" of club level because there was a chance we'd be a good candidate for future paid club level reservation. Second, we'd already paid to upgrade from the regular dining plan to the Deluxe dining plan, so again from the budgetary standpoint, chances are we would not use much of the "free" breakfast, drinks & snacks in the Club room. Which is true, we didn't use much so we were "cheap" to them on the upgrade. And 3rd, we were only going to be there for two nights - in the middle of the week. So no ever-popular weekend days tied up for the free upgrade. We were arriving the day after MLK JR. Day, so, the 3-day weekenders had already checked out. And we would be checking out on Thursday morning - before the next batch of 3 or 4 day weekenders would show up. Just my opinion, of course. Could be they use an entirely different set of criteria. Whatever, we enjoyed it.