And in your playbooks, what percentages do you have for each of the distances?
In a pass play, who the ball is thrown to depends on two things: who is targeted at a given distance in a formation, and where the ball is being thrown to for a given down/distance in the playbook.
In a formation, the numbers designate the percentages that a given player/position will be targeted by the QB. If you have your RBs at 5% for Very Short and Short, the TE is set to block, but the WRs are at 0, then your RBs will be the ones thrown to.
In a playbook, the numbers are the percentage chance that a pass will be thrown to a given distance - Very Short to Deep.