My understanding was that back in the old days, your M&M number was equal to your credit card number (if you had a German co-brand card) so the M&M numbers needed to be in the same format as credit card numbers.

I believe that this is also why the M&M system has your number change when your status changes, and uses a separate static customer ID to tie the various numbers together.

It is the only FFP I am aware of that uses this logic.