At the recent MOT of my 2007, MX5 with 23k miles on clock, (not franchised dealer) I was advised the front brake disks were scoured on the inside faces. The external (visible faces) are perfectly smooth, the brake pads are only 50% worn, the car passed the mot and the reason for the scouring was explained as possible result of parking next to grass. The car is kept in a garage, used mainly at weekends / summer (hence mileage) and has been owned for last 3 years and regularly serviced since new.

Is this possible, or to be expected with low useage and if so why only the inside faces? Is the explanation given plausable or an attempt to get me to spend more money replacing disks and pads? Any help or advice appreciated, safety is obviously paramount so if the work needs doing so be it.