The difference comes from the fact that the compensation ballast in the Commander is making up for the lack of an inboard motor and the weight of the extended cabin of the Ariel. Most PHRF systems give a different rating to each boat indicating that their weight (when sailed with the designed weight installed) isn't quite the same even with the installed lead. The somewhat old information in the chart on the Pearson Information page notes this:
http://www.pearsoninfo.net/info/phrf.htm
The Ariel scores an average PHRF of 255.5 and the Commander 250.5. I know that current numbers used by local sailing clubs on the Chesapeake are 252 for the Commander and 257 for the Ariel.
Sadly, nobody registered an Ariel or a Commander in the Chesapeake Bay Yacht Racing Association in 2008 (CBYRA runs the major races on the bay). A number of them ran in Wednesday night local races in smaller clubs. I intend to put my Commander out there in 09 in the non-spinnaker class. I don't expect to win but I'm looking forward to a number of good days on the water with some friends.