First, Pierce had a 4.3 YPC, not 2.3 YPC. Henry, Barkley and Cook all finished in the top 6 in rushing yards and all had a 4.4 YPC.
Second, you seem to recognize that he played in only 13 games (missed last four) but then assess his stats based on his ranking for the entire season.
Had he played all 17 games, based on his average over those 13 games, he would have had 1,230 yards. That would be good enough for 6th in the league, which is actually where he was tracking before he got injured. And that is with TWO games where he gained 8 yards.