Another factor is coaching philosophy: High scoring teams are built to maximize their output when things go well.
With Manning in 2010 the Colt's won 10 games. When Manning missed the 2011 season, they went 2-14. With new franchise quarterback Andrew Luck they bounced back to 11-5 in 2012.
With some teams, the difference between their first-string and back-up quarterback isn't that much, but if your number one guy is a superstar, its an entirely different story. One time, Jon Gruden and I were attending a Colts practice before one of our ESPN games, and we were standing next to their offensive coordinator, Tom Moore. Tom is 'old school' in every sense of the word. He's been in the NFL for over thirty years and has signaled in every play call of Peyton Manning's career. As we watched, we were surprised to see Manning taking virtually all the reps in the session. Jon asked Tom why he wasn't giving some snaps to Peyton's backups. Moore is a man of few words, but when he talks, thos ewords carry weight. He looked us both in the eye, paused for a moment, then said in that gravelly voice of his, 'Fellas, if "18" goes down, we're fucked. And we don't practice fucked.'
Really old school coaches like Don Shula built their teams for resilience when things went badly. Shula twice took teams to the Super Bowl when forced to play most of the season with his back-up quarterback. In his undefeated 1972 season, Earl Morrall, not Bob Griese, started a majority of Miami's 17 victories.