Jonas misinterpreted the Receiver's statement about his job (see below). Explain."My job is to transmit to you all the memories I have within me. Memories of the past."
I think you are asking how it is that Jonas misinterpreted this statement that the Receiver made.
The answer to this (from Chapter 10, page 101 in my edition of the book) is that Jonas thinks that the Giver is going to give Jonas memories from his own life. Jonas tells the Giver that he would be honored to hear about the Giver's life.
It makes sense that Jonas would think this because, after all, how can anyone have memories of something that did not actually happen to them? Of course, as we find out, the Giver's memories are not like real memories that we have of things that happen to us.