When Ender comes into his dormitory he sees that the other nineteen kids in his launch already took their beds leaving only one bed for Ender:
The other boys had already chosen their bunks when Ender arrived. Ender stopped in the doorway of the dormitory, looking for the sole remaining bed. The ceiling was low Ender could reach up and touch it. A child-size room, with the bottom bunk resting on the floor. The other boys were watching him, cornerwise. Sure enough, the bottom bunk right by the door was the only empty bed. For a moment it occurred to Ender that by letting the others put him in the worst place, he was inviting later bullying. Yet he couldn't very well oust someone else.
But then when Ender takes the last bed, Dap says that they usually put the launch leader there, but now it's too late to recode the beds.
This room is your home for the first year or so here at the Battle School, so get the bunk you want and stay with it. Ordinarily we let you elect your chief officer and install him in the lower bunk by the door, but apparently that position has been taken. Can't recode the lockers now.
If they only had the exact number of beds either way, then how would they ever be able to put the launch leader there without recoding the lockers?