Why is Mr. Underwood's editorial defending Tom Robinson surprising?

Mr. Underwood's editorial is surprising because it suggests a change of opinion, even a major character change on his part.

Mr. Braxton Bragg Underwood runs Maycomb's only newspaper, The Maycomb Tribune. His most notable action in the story, prior to writing the editorial, was hiding out in his office, covering Atticus with a shotgun when the lynch mob approaches the jail to kill Tom Robinson. Atticus remarks the next morning that these actions are surprising, as Underwood "despises Negros, won't have one near him" (Chapter 16). Already, Underwood seems to be undergoing a change of heart and weighing his racism with his sense of justice. This internal struggle seems to have some resolution when Underwood writes the editorial. It seems that at least one man has had his prejudices challenged and changed during the course of the novel.