Other mistake: In Tiffany's converted garage, Tiffany says she's thinking of putting in a ballet bar, but there's one there already right next to the big mirror, where it's supposed to be.
Other mistake: When DeNiro is discussing a bet on a future (7 weeks away) game between the Eagles and Cowboys, DeNiro points out that the Eagles are favored by 1.5 points. In fact, betting lines are not set on games until the previous week's games are played, because there may be a major injury which would affect the line. As a bookmaker, DeNiro would certainly know this. And bookmakers are rarely fans of teams, and it would be unusual for a bookie to be such a rabid fan as DeNiro is in the movie.
Chosen answer: Their score was pretty bad, but just good enough to win the bet. Getting the money was the important part - they didn't really expect to win the competition.