Luck can play a huge part to the success of any NFL team. Injuries to players often happen as a result of bad luck; a wrong step on a poor field, a quick plant where the cleats dig in and doesn't let the foot go, or just simply being in the wrong place at the wrong time. Those team that can manage to stay relatively healthy often have better results on the field and in the W-L column.
Just as important as staying healthy is continuity. Having consistency amongst the players, coaches and schemes all play a major role in the success of a team, and this aspect of the game is something that often goes unnoticed by many when projecting how a team may or may not perform throughout the season.