Actually, there is no "national pool." . . . There is no "fill in the blank" process at the end to give out the rest of the slots. If there are slots unfilled, it is just a smaller class that year.
As others have said, this is completely incorrect. Those who do not win their slates go into the National Pool. The class is filled from that group. It is true that the number coming from the National Pool has decreased for USNA in recent years. Two reasons. First, USNA has worked hard on Admissions outreach to get qualified candidates from every district whereas in the past, some districts had no candidates, let alone qualified candidates. Second, USNA's entering class size has decreased due to reduced attrition.
Thus, your best shot is to win your slate. However, if that fails and you're an outstanding candidate, you still have a shot. USNA always fills its class -- they don't take fewer plebes than authorized.
There may be a different approach for other SAs . . . need to discuss that on those forums.
As for why WL folks have a good chance if reapplying . . . you are SO close. You are one who is waiting in the wings to get an appointment if yield is lower than anticipated. You might still be appointed. But, if not and you knock it out of the park as a college freshman, you have a good shot. As an aside, if you are WL'ed, I strongly suggest you go the college route vs. prep school. If you are WL'ed, your application is not deficient; rather, you're likely a victim of the numbers game. Too many great candidates for too few places. Thus, you likely don't need additional academic help, study skills help, etc.
You do need to go to a 4-year college (if you can afford one), do really well in the plebe courses, take on leadership as able -- read the sticky above on reapplying. Also keep in mind that many of you will absolutely love where you end up -- which is a great thing!