If you took a look at the Total QBR rankings, one thing you might notice is six quarterbacks packed together in a small range from 68.4 to 69.8. This would lead one to believe that these quarterbacks have had roughly the same level of performance this season, making them basically indistinguishable on a statistical basis.
A deeper look tells a different tale. Joe Flacco has put up a 69.7 Total QBR against a relatively tough slate of defenses that includes the Eagles and the Browns, while much of Philip Rivers' 68.4 has come against the porous units of the Raiders, Titans and Chiefs. Account for this difference and adjust their performance ratings accordingly and you get a clearer look at which early performances stand out -- for better and for worse.
After adjusting for the quality of defense faced by each quarterback, you can see why Flacco is clearly the better of the two QBs, why Ryan Tannehill is having a better start to the season than Cam Newton and why Brandon Weeden's debut may be as bad as it appears.