What made Jem worry that the jury wouldn't immediately come back with a not guilty verdict despite the facts given?

Reaction to the Trial

Tom Robinson’s trial is a watershed moment for Jem’s character. Throughout the trial, Jem watches with great interest, and is convinced that based on the evidence, there is no way the jury can convict Tom. So when the verdict comes back as guilty, Jem feels as though he’s been physically attacked.

Judge Taylor was polling the jury: "Guilty... guilty... guilty... guilty..." I peeked at Jem: his hands were white from gripping the balcony rail, and his shoulders jerked as if each "guilty" was a separate stab between them. (21.50)

While Jem’s certainty about the trial’s outcome is receiving these blows, the verdict also seems to be a broader attack on things Jem thought were true: that the legal system is just, that innocent men are acquitted, that Maycomb is a community of good, fair-minded people. After the trial, Jem struggles to figure out why people are so eager to divide into groups and hate each other. Scout says that people are just people, but Jem isn’t so sure.

"That's what I thought, too," he said at last, "when I was your age. If there's just one kind of folks, why can't they get along with each other? If they're all alike, why do they go out of their way to despise each other? Scout, I think I'm beginning to understand something. I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time... it's because he wants to stay inside." (23.117)

The Tom Robinson trial makes Jem lose his faith in humanity. Will he ever get it back? Is there a way to acknowledge all the evil people do and be able still leave the house? (Atticus might have something to say about that.) Jem is unconscious for the conclusion of the novel, so he doesn’t have the same moment of revelation that Scout does, but perhaps his waking up will also be a kind of rebirth.

Actually, I don't think Jem WAS worried. He looked logically at all the evidence. There's a description of him saying, "We've got him," after Bob Ewell's testimony. He of course does not consider the racial element, only the cold facts. He cries after the trial--which Scout does not.