I agree with Jemach here.
I remember reading some of pff's "statistical" work and was not impressed. That work would not have stood scrutiny in a professional environment.
I think because so many people love football they naturally delve into many things that help understand the game. We all do to some degree or another. But it is a egotistical for someone to present himself as an expert in stats because he's read a few articles and is able to do some algebra.
With the game of football, it is not difficult to make some stats make a point for you. I think most of the sites that present their "stats" work as articles should be taken with a large grain of salt.