When looking at a running back's per-carry average, it's important to remember how they're used. Powell (4.0) is a situational runner, so his average will be higher. Greene (2.8) is used in short yardage, goal line, the four-minute offense and he faced 10-man fronts on his two runs before the game-winning FG in OT.
Let's compare the backs on a level playing field. On first-and-10 runs against the Dolphins, Greene averaged 3.6 (seven carries for 25 yards) while Powell averaged 3.4 (five for 17). Yes, Greene has some poor reads, but the team, looking at the big picture, intends to remain patient.