Marty has always been portrayed as being extremely intelligent, he is highly educated and working in computers and science, clearly the smartest character of the main cast. When the delivery guy knocks on the door, Calvin tells him to answer the door, then yells at the door "Stay right there" and "Don't move" (while facing the door and not Marty), things clearly directed at the delivery guy, intended to prevent him from leaving prematurely again. Yet Marty still mistakes these comments as being directed at him, a literal moment after Calvin told him to get the door. It is not reasonable that a person as intelligent as Marty would make such a silly mistake.