Matthews was pretty much the run-away candidate for the award at the beginning of the year.
He recorded 8.5 sacks and a forced fumble in his first 5 games. Since then, he has come back down to Earth and only recorded 5 sacks in the remaining 11 games, which is not bad.
The thing that strikes me most though, is that he is a pass rusher with only 2 forced fumbles on the season.
When you compare his numbers to Harrison, or any of the well-rounded guys with larger sack numbers, I do not see how you could put this guy above them.
Moreover, it is rather revealing when you consider nearly half his sacks came in the first two games of the season.