Why is number 19 skipped in the Assignments in The Giver

Jonas is skipped. Jonas is stunned and hopes wildly that someone has made a mistake, but even his group leader looks worried. Ashamed, he sinks into his seat and wonders what he has done wrong.Jonas of course is being saved for something special. After the applause dissipates, the Chief Elder apologizes graciously to the community and to Jonas in particular for causing them anxiety, and she calls him to the stage. He walks clumsily to the stage, and she tells him that he has not been assigned because he has been selected to be the next Receiver of Memory, which astonishes and awes the audience, although he does not understand.