How the rest of the league works is irrelevant because what Cam did in 2015 is an outlier, something that he wont ever repeat.
Let's review the facts of his 2015 season:
19.6% of opponents drives ended in the turnovers for the Panthers. #1 in NFL.
12.4% ended in Interceptions #1 in NFL.
They held opponents to 2:21 TOP/DR #1 in NFL.
Panthers had #2 average offensive starting field position.
the defense consistently put Cam and the offense in GREAT position to succeed. No offense in the league had it better than Panthers.
On top of that Carolina faced the easiest defensive schedule in the league last year, including the WORST defensive team in history twice(Saints).
All of that leads to inflated touchdown numbers for Cam and inflated scoring numbers for the offense in general. Cam had a good to great year, but he didn't deserve MVP and he wasn't the best QB in the league like many proclaim.