According to Kodak the difference is in the overcoating.
All film has a thin coating of gelatin over the emulsion to
protect it from abrasion. Since the developer must penetrate
this layer it affects developing time a little. Some of the
differences in the charts are simply from retesting since
reportedly T-Max films have actually been made in the new
facility for some time.
 As far as performance there seems to be no difference.