How do the events of the final chapter of "To Kill a Mockingbird" explain the first sentence in the whole novel?Chapter 31 To Kill a Mockingbird

In the first sentence, Scout says that Jem had his arm broken when he was 13.  She doesn't really mention the rest of it until the end of the book, and the encounter between the Finch children and Bob Ewell.  Once the novel concludes, we understand that Jem broke his arm in the scuffle with Ewell, as he tried to protect Scout from Ewell and escape Ewell's attack.