If the server is 21/21, then its a complete full server, nobody else can come in (even if you have slot reservation, there's just no room physically). The server will then kick someone to make it a 20/21 server (Dropped due to slot reservation). The server will try to keep it a 20/21 server so when anyone with slot res comes in, there is room, and then the server will kick another player making it a continuous 20/21.
This is from my understanding though.