I would recommend you read the following information about how changes in turnover margin from one part of a season to another is completely expected within teams throughout the league:
http://blog.minitab.com/blog/the-st...look-at-how-turnovers-impacted-the-nfl-season
As for Belichick for example, when he was with the Browns from 1991 to 1995, his teams' overall turnover margin during those seasons was -5.
Belichick indeed stands out from the crowd with New England, however, as his +171 turnover margin during his tenure there dwarfs the league average during that period. But even then, New England's turnover margin during those seasons has varied all the way from -6 to +28. In seven of those seasons their turnover margin was at least +10, and in 10 of those seasons it was less than +10.
Why all that variation, if the explanation is simply that Belichick is so good at coaching with regard to turnovers? Why don't the Patriots simply enjoy a turnover margin between let's say +15 and +20,
every year? If coaching were the only cause of their success in that area, one would expect that sort of very stable and predictable performance, rather than loads of variation from year to year.
I'll tell you why: because per the link above, the only component of turnover margin that varies even somewhat systematically is interceptions thrown, and the Patriots have led the league in that area during Belichick's tenure. Now is that a function of Belichick, or Brady?
Even if we were to attribute Belichick's overall turnover margin in New England completely to coaching, we certainly can't attribute the Dolphins' change in turnovers during 2016 to a similar stable coaching trait of Adam Gase's, when we have nowhere near the sample size for Gase, and turnovers vary greatly from one part of a season to another throughout the league, even within teams, per the link above.
See if you can find someone here who's willing to bet for example that the 2017 Dolphins will have a turnover margin of at least +10, or even +5. Anyone would be a fool to make such a bet. They'd be similarly foolish to bet that the turnover margin will be worse than -5, or worse than -10. We simply don't know what's going to happen with any precision.
And why is that? Because turnovers vary largely randomly and not systematically, which makes it very difficult to predict with any precision what any one team is going to do in that area in any one year.