While the camera focuses on Mike and Robbie painting the fence, you can hear Bub, Kincaid and Verna discussing sandpaper in the background. Then you hear a new voice talking about a guy on 8th street with an electric sander. When the camera shifts to Bub and his group, there are still only 3 people there, Bub, Kincaid and Verna. Then the milk truck drives up and the milkman comes over. Verna asks the milkman about sandpaper and he starts talking about a guy on 8th street with an electric sander. It was his voice you heard in the background earlier, but he hadn't physically shown up at the scene yet.