Watch the the video, you will see why this was acted out.
http://www.youtube.com/watch?v=eupEkeYOhj4
I do find the explanation in the voice-over convincing, just based on the simple realities of human behavior, especially in regard to the “girl on the right”.
Reminds me of scene in The Godfather when the supposed capo gives himself away by glancing at the real boss for approval, as Brando later notes, saying, “I never knew until that moment ... “