Top of my head, I have a couple of thoughts.
1) Pace isn't an official stat, so it is sometimes approximated by deriving the number of possessions from made shots, missed shots with defensive rebounds, and turnovers. This is especially true going back in time, when play-by-play data doesn't exist. Some sites may use that method for all their stats, to be consistent, whereas others may use play-by-play data for current year stuff.
2) Overtime games can mess up your points per game numbers, since pace is normalized to 48 minutes.