I think much of it has to do with the flow of the game. It's boom or bust with Dak. He's REALLY streaky. It's not abnormal to see a stat line that reads 12/20, 150yds, 0 TDs, 1 INT through 3 quarters of the game, and then a 4th qtr line of 10/12, 200yds, 3 TDs, 0 INTs in the 4th, giving the illusion that he had a great game.
That final stat line doesn't account for the numerous 3-and-outs, big sacks taken, fumbles lost in either redzone, etc etc. When the offense performs poorly, unless you have the 00 Ravens or 85 Bears defense, there's a pretty good chance the D was worn out at halftime, and a poor second half performance is inevitable.
On the offensive side of the ball, I think Rush is just better at the mental side of the game. My guess is he making better adjustments at the line, calling proper protections, checking out of bad pass plays and putting Dowdle in a better position to hit the open gaps. Rush is playing within the boundaries of the offense and limiting hero ball, allowing for a more consistent performance and keeping the defense of the field long enough to catch their breath...which of course allows them to remain effective throughout the game.
Maybe I'm wrong. If I am, somebody please tell me why the defense and run game are suddenly better with Dak on the sideline.....
