Ok.... how about there have been 10 seasons since 1970 where the NFL ypc > 4.1. 9 of those have been since 2002. Also, the chart did not include seasons > 2012. I suspect with Dallas. Philly and Seattle running the ball like they have, the tenths continue to stack up over the last 2 seasons.
It is a statistically significant variance.
When I see those numbers I think immediately think of two reasons why the YPC have creeped up over the last few seasons:
1. More "running" QB's in the league. Typically "running" are poor passers, but usually have strong YPC averages, which of course positively influences the overall YPC average of the league.
2. The heavy passing nature of the league forces many defenses to play in the "nickel" (like Dallas) or some other defense that takes "weight" off the field and puts "speed" on it. The result is when the offense does run the ball, there's not only less "beef" to stop it, the defense is spread out more-so across the field so there are less defenders in the box in order to stop the running game.
In my view that doesn't mean a "renaissance" for the running game across the NFL, but a combination of more QB's that like to run and defenses are picking their "poison" and it's the pass.
Now that being said and an offense is going to become a predictable running team then the defense is going to start respecting that and the YPC will decrease like what we saw over the last quarter of the season with the Cowboys. There were still run heavy and defenses were putting 8-9 men in the box and running a variety of run-blitz to stop the run and consequently the YPC averages for the last quarter of the season were not as high as previously experienced.