They're both thinking, "Darn, we could have gotten a Model T and been a lot better off!!!"
Time exposure. They were both bored with the wait. It is harder to hold a smile for a minute or two than to relax the face for the same period. Try it! The photographer likely had to set the camera then put his head under a black cover and squeeze a bulb. Meanwhile, they didn't know quite when he was starting to take the actual picture or when he was done, so they just sat there very still.
Norm