The answer to this question can be found in Chapter 21. At the beginning of the chapter, Jem pleads with his father to be allowed to return to the courtroom to hear the verdict. Jem asks if his father thinks that the jury will "acquit [Tom Robinson] fast." Jem...
The answer to this question can be found in Chapter 21. At the beginning of the chapter, Jem pleads with his father to be allowed to return to the courtroom to hear the verdict. Jem asks if his father thinks that the jury will "acquit [Tom Robinson] fast." Jem later tells Reverend Sykes, "Don't see how any jury could convict on what we heard." Jem is clearly expecting a "not guilty" verdict to be returned, as of course it would be if the Maycomb County jury were genuinely impartial. He does not think the jury will be biased because of their racial prejudices. Reverend Sykes warns Jem not be so sure about a just outcome, telling him that he "ain't ever seen any juury decide in favor of a colored man over a white man."
When the verdict is read out, and each jury member is proclaiming Tom Robinson to be guilty, Jem becomes visibly shaken. His hands are "white from gripping the balcony rail, and his shoulders jerk ... as if each "guilty" (is) a separate stab between them." The verdict clearly takes him by surprise. After the verdict, Jem cries. His face is "streaked with angry tears,"and he mutters to himself over and over again, "It ain't right."
In simple terms, the difference between Jem's expectation of the verdict and the actual verdict is the difference between a not guilty verdict and a guilty one. The more meaningful difference, however, is the difference between Jem's understanding of people and society, and what people and society are really like. Jem believes that people are fundamentally decent and fair, and that society, at least in matters of life and death, is just. When the verdict is read out, Jem begins to learn that people are not as decent as he thought, and that society is not as just as it should be.