I think you cherry-picked that list a bit. Of the 13 teams with winning records, only one, the Patriots with Sammy Morris, have a leading rusher with a lower ypc than Lynch. And only two others, the Cards with James and the Bears with Forte, have leading rushers under 4.0 ypc. The Ravens' leading rusher, McClain, is over 4.2.
Now compare Lynch to Jackson. Jackson, for the second straight year, is a full ypc ahead of Lynch. And even though there's a perception that Jackson is a smaller, change of pace back, he's listed at the same weight as Lynch (I'm sceptical of that, but it's true). Jackson doesn't seem to take the same beating as Lynch, and, unlike Lynch, he hasn't missed a game due to injury. Yes, maybe if Jackson were put in on more 3rd and 1s his ypc would drop, but maybe not.
And finally, just by the eyeball test, hasn't Jackson seemed like a more effective runner this year?