Both systems are reporting full, they are actively managed by both parties to assure that the groups don't oversell. There's a lot of banter about one system vs the other letting you in, but that isn't the case.
What "does" happen is if someone cancels, an extra slot will become available and an email notification will go out. First to respond and sign-up gets the slot, so react quickly if you see an N2 alert email. It does happen all the time, people get called into work, etc. Oh, and if it rains, it's amazing how many trucks break down or unexpected funerals occur on Friday