6 for 6

I am fascinated by this chart which appeared in yesterday's NY Times graphing the average points scored per NFL drive for drives starting at each yard line.

Understandably, drives starting at the opponents 1 yard line are the highest scoring starting points. But the next highest scoring starting position is not the 2 yeard line but the 7 yard line?!

What could account for the difference? Either the extra yards allow teams more/better passing options by giving defenses more territory to defend, or the 6 yard line gives offenses more options and defenses, conversely, more things to consider than if both teams were focused on running it in from, say, the 3 yard line.

Moreover, teams score more if they start at the 8 yeard line than if they start at the 3, 5, or 7 yard line.