I always thought that the idea behind mystery shopping was to see what the customer service was like when no one was watching. I've done a few restaurant shops, and 35 mph food shops (the ones you have to go inside, to get the food but it's still kinda fast).
I have seen the difference between the days when regional managers are in the store and when it's just the store managers. It really shouldn't be that way, if you want the job you should be doing it the right way every day that you're there, no matter what or who is watching.
In the way back machine, I had a produce guy at a grocery store who apparently had a friend or family member who was shopping their stores give him the questions every month. I contacted the MSC and they said because of this, and for no other reason, I was to make up a similar question, on the same subject of the original question when I talked to him. Unfortunately there was some kind of shake up at that MSC, and the folks that are running that grocery shopping program now have no concept of loyalty, or the effort we put into doing the job.