Well Gale has Return specialist ability, that alone might carry some bonuses here and there that gives him more speed, agility and skill carrying the ball.
As for Emmitt and Earl, Emmitt when I played with him he was faster and more agile with not as much power. This could be due to size, if a player is too big they lose out in agility and speed but gain in power and strength. In 2k5 bigger players in weight got better overall ratings even with the same exact individual skill ratings. I saw this when I created Brandon Jacobs in NFL 2k5.
I'm not saying I am right, just making some observations.