Hi Cruella
To be truthful I have no idea if the DWP have your award expiry date wrong on their system, it may be possible, the only way to know for certain is to contact them, it can take a while to get through to them.
Extension letters are automatically being sent out to claimants, although I have not heard from any members on the forum of their extension date being sooner than the actual award length expiry date.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems