Different mortgage-backed securities with the same issue date, same coupon and same original face value can have greatly different current faces. Mortgage-backed securities pay down at different rates based on the characteristics of the underlying loans and on the actual prepayment speed of underlying mortgages.
For example, suppose that two mortgage-backed securities (MBS 1 and MBS 2) have the same original face value, but MBS 1 experiences faster prepayments then MBS 2. In this case, MBS 1 will have a lower current face value then MBS 2 as time progresses.