I don't know if NBA only lists top categories for the superstars, but Pierce is like in the top 20 in a lot of categories. Terry is not listed in the top 20 in any category. I would think that the statistics are generated by the computer not people on NBA.com.
Compare Pierce to Terry on NBA.com player profiles. Unless Terry is getting the shaft, Pierce is of the caliber of Dirk because only Dirk on the Mavs has similar top 20 statistics.
Terry
http://www.nba.com/playerfile/jason_terry/index.html
Pierce
http://www.nba.com/playerfile/paul_pierce/index.html