Bad coaches perform fairly well when everything is lining up great (health, schedule etc) and perform terribly when they don't. (ex, Dennis Allen)
Good coaches perform great when everything is lining up great, and perform fairly well when they don't. (ex, Mike Tomlin, Mike McCarthy)
The best coaches perform exceptionally well when everything is lining up great, and perform fairly good when they don't. (ex, Andy Reid)
These are the differences between non-playoff teams, playoff teams and superbowl contenders.