Like many stats thats misleading. Giants defense had injuries during the season, when they got healthy at the end they played very good. Big part of the reason that made it to, and won, the Super Bowl. I think they have one of the better defensive lines.
You have to win games during the season to make the playoffs. They won enough games with a bad defense during the season to get in. That's the point. Better example, the Pats defense was ranked 31st last year. If Wes Welker catches that pass in the 4th quarter they WIN the Super Bowl. That's the point. You can get by in this league with a sub-par defense. You CAN'T get by with a sub-par offense