It can be only one reason. The field is lighted more than the stands. No camera can compensate for different lighting conditions. It is possible to photo shop a photograph.
I thought the same thing at the start of the game. I concluded that lots of fans were wearing their heavy coats to stay warm and most of those coats were black.
Compounding problems was that godzillamegatron was dark for a chunk of the 4th quarter...and I have to guess that is not something the guys in the truck ever planned to have to compensate for in a night game.
The north end zone upper deck is dark from my luxury seats in section 103. From our vantage point you'd think they needed a reading light to see the programs.