What does Jem misunderstand about the length of the jury's deliberations? What is the significance of the jury's deliberations, and what effect do they have on Jem?

In Chapter 23, Atticus is discussing aspects of the trial and justice system with his children. After Atticus tells Jem that serving on a jury forces a man to make a declaration about something, Jem says, "Tom's jury sho' made up its mind in a hurry" (Lee 136). Atticus immediately corrects Jem by commenting that Tom's jury took a few hours. Atticus felt that Tom had a slight chance of winning simply because the jury was deliberating for such a long time. Atticus tells his son that in an inevitable verdict, it usually takes the jury several minutes to deliberate. Atticus then explains to Jem that one of the Cunninghams was actually arguing for an outright acquittal. Jem is both shocked and perplexed to learn that one of the Cunninghams argued for Tom's innocence. 
During the trial, Jem did not understand that the jury was inevitably going to convict Tom Robinson because he was black. Jem naively believed that Tom would be given a fair trial. However, Atticus understood that typically a trial involving a black man's word against a white person's word takes a few minutes. The fact that the jury deliberated for a few hours is significant because at least one white juror supported Tom's testimony. Having a white juror argue a black man's innocence in 1930s Alabama was unheard of, which is why Atticus felt somewhat optimistic. Upon hearing Atticus' explanation, Jem mentions that he'll never understand the community members of Maycomb.