One way is taking the highest point of all the components individually during the day, regardless of where other components are trading. So the "high" is not necessarily the highest point that the average hit collectively at the same time.
I'm not sure if that is the reason for the difference in this case or not.
Maybe that is the explanation, the highest point for the actual Dow to have hit in real-time, as compared with the individual Dow components highs for the day, all weighted to come up with a theoretical Dow high point for the day.