I have wondered the same, I do wonder if the cards have more digits to help airport locations know if a user can use the card before they enter. For example, if you went to a restaurant and tried to your an Amex issued PP card (that does not have the dining benefit assigned) the host could quickly tell if the card was not eligible, where as a Chase Reserve card would have the benefit and also more digits to distinguish it.

It is unlikely this is the only reason but it might be an added benefit to the longer number.