I may have figured out what happened. I just won this too - and I took care to read the winning screen very carefully so I would not have the same mixup with my prize.
the screen says: CONGRATS, CHAMP. YOU JUST WON A FUNNER-TO-THE-FINAL-SECOND NFL PRIZE"
It goes on to say, it will let me know WHAT PRIZE I have won in an email.
Perhaps when you saw the words "second NFL Prize" you took that to mean, Second prize.
If that's not what happened, and your winning screen clearly said "You have won second prize" - then it sounds like they have changed the wording of the winning screen, to let people know they will be notified of the prize in an email. (Perhaps yours wasn't the only erroneous screen.)