I believe it has to do with who checks in first and makes a request. I know there have been very few times my requests have been met when I first check in. I usually very politely mention what my room requests were and ask if I got them. Usually I haven't, but the CM checking me in always says something like "Let me see what I can do." I have heard them, more than once, say something about switching someone else to another room, then laugh. I usually get close to what I requested. Of course, I'm not that picky. In a deluxe resort I ask for top floor close to elevator. In a mod or value I ask for close to food court and buses. I can probably count on one hand the number of times I've received the room I was actually assigned prior to check in.
What I'm trying to say is that if someone checked in ahead of you with the same room requests, there is a good chance that if they reiterated their request nicely at check in, that they may get your room, while you get bumped. Then if you check in before someone else and reiterate your request, someone else may get bumped.