When the bag was first designed it was named after Sofia Coppola (MJ's original muse!) and the correct name of the bag was S-O-F-I-A. The spelling on the tags changed during the second season - I don't know if it was a mistake on the printer's part or if the spelling was purposely changed. I know that it was constantly being mis-spelled (by various stores, online descriptions, etc.), so maybe they just decided to make it simpler and changed the spelling to S-O-P-H-I-A.
^^^That is my understanding too: SOFIA is the original and correct name, after Sofia Coppola, and SOPHIA is a typo that appeared so frequently on dept. store tags from NM, Saks, etc., that it became widely accepted.