I can't see how scratches on a mirror could interfere with the meter, as long there is no substantial change in the overall reflectivity.

In those cases where the meter is located behind the meter typically only a fraction of the light is sent to the meter, so a smaller change in overall reflectivity would already affect the meter.

But still I cant see how scratches could be of affect. Maybe aside of the visible scratches a seemingly even abrasion has taken place.