Wharton's invitations are released at random, depending on which which readers read faster, which applications happen to be printed out first and which last, and a whole host of other unsystematic factors. Applications are not separated into geographical area or any other demographic, and receiving an interview invitation early is not an indication that your application was stronger- it just means that it happened to have gone through the process quicker than everyone else.
In sum, relax. =) I personally received my interview invitation only in the last week.