Jem is older, so he understands more about the trial than Scout does. At Jem’s age, he is very interested in justice. He is feeling more adult, and he wants to experience things as an adult. He is convinced that Atticus is going to win the trial.
When Atticus tells Jem be thinks the jury won’t deliberate long, Jem’s response is telling.
"You think they'll acquit him that fast?" asked Jem. (ch 21)
Jem saw the trial. He understood that Atticus was proving that the crime was not even committed and that Tom Robinson was crippled and could not have committed it any way. He assumes that this means that Tom will be acquitted.
Scout does not really know what to expect. She is younger, and understands less of the process than Jem. She realizes that Atticus has proven Tom’s innocence, but does not know enough to expect him to be acquitted.
The verdict is a harsh blow for Jem. He expected the world to be fair, and he finds out it isn’t.
Jem is also at an age when he really looks up to and admires Atticus. He even says he wants to be a lawyer. He sees the trial as law school. He thinks that lawyers work in a world of fairness. The guilty verdict squashes those dreams too.
It was Jem's turn to cry. His face was streaked with angry tears as we made our way through the cheerful crowd. "It ain't right," he muttered, all the way to the corner of the square where we found Atticus waiting. (ch 22)
Jem gets a harsh lesson in how the world really works. Atticus agrees with him that it is not right. He says he will try for an acquittal, but he is not confident. He knows that racism is an insurmountable battle. This is the lesson that Jem is learning, but Scout only understands that Jem is upset.