This came up in another thread. The change in the middle digit is connected to a change in status. The last digit is likely just a check digit, which would naturally change when any of the earlier digits change.
As for sending in ID to register for a promo, maybe your account got frozen like mine did, which prevented me from redeeming points. Account was frozen when the "system registered a sign-in from an unknown device", according to Accor. Needed to send ID and change my password to unlock the account.