Jem misinterpreted Atticus's statement, "But I expect it'll be over before you get back". Explain

When Calpurnia arrives, Atticus realizes that his children have been in the courtroom all along. He gathers the children and sends them home for dinner, promising Jem they could come back to hear the verdict. Atticus adds that he's sure "it'll be over," before they return to the courtroom. Jem misinterprets Atticus' words and replies,"You think they'll acquit him that fast?"