Did you look at total # of 1st downs gained by reggie / number of carries?
Because YPC is an average of how many YARDS someone gets PER CARRY.
So "on average", if you give him 3 carries, he will get 10.5 yards, and a 1st. Not sure how youre saying this is incorrect? This is an AVERAGE, not a predictor of how many 1st downs he statistically has.