I was fortunate enough to be around some tremendous coaches during my time as an NFL player and scout. One thing I learned from guys like Jon Gruden and Bill Belichick is the most successful teams are the ones that don't consistently beat themselves.
Another thing I learned is that depth is extremely important. Belichick especially stressed quality depth at certain positions because those positions allowed him to run the schemes he wanted even in case of injury.
These are two of the biggest factors that go overlooked during an NFL season. We can analyze 53-man rosters at the beginning of the season and think we know which teams are going to be good and which aren't.