each year people end up dropping an not taking the commission for one reason or another, if they need 50 people for DCO Intel and select 50 then if someone drops they will miss the goal, so they often will select 50 and say 5 alternates as an insurance policy, in some cases the alternates will be designated selections for the next FY, but often they go back into the pool to reapply.
The other things they have done is over-selected to ensure they get who they need since they just have a guess as the actual goal, in that case if they think they need 60, but pick 80, then actual goal is 50 they will then slide those to the next FY, or they will issue a "no quota" letter which means yes you were selected but sorry we don't have a spot next year so reapply.
I saw many DCO applicants get a "no quota" letter in the past due to the DCO boards often over-selecting, the board could be held at a better time in my opinion to minimize this.