Some of that can be dictated by style of offense. Hightower in a run first offense without a good QB means 8 in the box a lot. Thus lower YPC. Put him in an offense with a threat @WB and he's got bigger lanes and more room to maneuver.
Sometimes u have to ask why..instead of just stating the obvious. Similarly Ingram would prob Me more effective in a run first offense because he's a volume runner.