I hadn't thought about this before (or the penny hadn't dropped)....
Car parks A (and presumably B) do have ANPR but they can't be used for match days as the exit ANPR gate is located in a different place to the entrances.
On a weekday if you go into Car park A you enter via the north stand area.. to Exit A you then drive all around the stadium (passed Stands E, S, W) then through a gate to the North west of the stand.
On matchdays this can't be done because of the crowds walking on the service road around the stadium and presumably the emergency vehicles and tv vehicles and various temporary barriers also block that off.
If you park in Car park A try and park in the far North east corner as that's where the temporary exit is