In "To Kill a Mockingbird", why does Lee select a Cunningham to be a juror who cast the not-guilty vote?

Cunningham recognizes in Tom Robinson his own disenfranchised status in the town.  He can relate to the prejudice against Tom's race, having experienced the same prejudice against his own class.  Scout's recognition of Cunningham in the lynch mob would have reminded him of his connection to the Finches through the kindness to his son, and to himself, in former days.  The novel's shifting contrasts between characters, emphasizing on one hand their differences (as when Cunningham is part of the lynch mob) and their similarities (as when Cunningham is reminded of his lowly status) is a theme borne out by Lee's choice of Cunningham as the one holdout in favor of Tom Robinson's acquittal.  When Cunningham walks in Robinson's shoes, he can fairly judge him.