How does Atticus explain the jury's actions to Jem in Chapter 22 of To Kill a Mockingbird?

Jem is devastated when the jury gives Tom Robinson a guilty verdict. He had clung to the hope that jurors would look at the evidence and declare Tom Robinson innocent. Jem had faith in the jury to do the right thing, not the popular thing.

At home, Jem cries at the injustice. Atticus is solemn and heads up to bed early. Jem asks his father how the jury could have convicted Tom Robinson. Atticus responds,

...

Jem is devastated when the jury gives Tom Robinson a guilty verdict. He had clung to the hope that jurors would look at the evidence and declare Tom Robinson innocent. Jem had faith in the jury to do the right thing, not the popular thing.

At home, Jem cries at the injustice. Atticus is solemn and heads up to bed early. Jem asks his father how the jury could have convicted Tom Robinson. Atticus responds,

"I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do it—seems that only children weep" (Chapter 22).

Atticus is referring to the fact that juries always convict black men if they are accused of a violent crime against a white person. This is the way things are in the South in the 1930s. It is considered socially unacceptable to do otherwise. Atticus does not know exactly how they can do such a thing after examining the evidence that proved Tom's innocence. He just knows that this is the way it always goes.