Other mistake: At the beginning after Dunbar leaves the surgery area, he walks up to fence where a soldier is lying down. Next to him is an apple with the grocery sticker on it. (04:49:00)
Other mistake: Dunbar has to race back to the fort in the middle of having a shave. The amount of soap on his face alters shot to shot.
Chosen answer: It shows that the officer was mentally disturbed, and he was the only one in the fort who knew about Dunbar's assignment. It sets the story up so that Dunbar could live with the Indians without the Army interfering with his life (No one expected any communications to or from Dunbar).
Twotall