That's really not the case. Before the national pool, you're competing with the other candidates on your MOC's slate. The reason that most appointments come out in March is that it's shortly after the application deadline. Each MOC can nominate 10 people for each slot they have, but only one will get the appointment (at least charged to that MOC). If a principal nomination, they will be offered it as soon as their file is complete and they're qualified. On a competitive slate, the academy chooses the most qualified candidate, which they can't do until all 10 applications are complete. So even if your application is done and you have the nom in November, the academy can't compare you to everyone else until they're all done - which has to be done by March 1. Assuming time to process, this is why so many appointments come out in the "mass mailing" then.
If you're not the most competitive on your slate (or are on a slate with a different principal nomination), then at this point you'll go on to the National Waiting List. According to my MALO training, WP at least MUST take a minimum of 150 off the NWL, in WCS order and offer them appointments. After that, to fill the remaining slots, it says they MAY take them in WCS order or not. So if they have 50 more slots (I have no idea on this next number) and want another person from NJ or a tennis player or a Pacific Islander or whatever - at that point they can fill by preference. The first 150 are by WCS score though.
So during this same time, if you're nowhere near the top of the NWL (ie would have no real shot in getting an appointment), you would then get a rejection letter. If you have a reasonable WCS score, you probably wouldn't get anything. If your score is low, but you're high on your slate, you probably still won't get anything while they wait to see what the #1 person does.
This is where people talk about hearing in May/June (my WP roommate was notified less than a week before R-day). This comes about as a result of someone who was offered an appointment waiting until the deadline (May 1) and then turning it down. Or getting injured after that day. Or whatever reason. They then offer it to the next person - off the slate if they were charged to a MOC or off the NWL if they were from there.
Rolling admissions simply means that if they can offer you an appointment (ie you have an LOA or all 10 applications are complete or whatever) that they do it right then, not wait and send them all out in March.