To me, nowadays 5 stars means they did not mess anything up. Or maybe just one minor thing. Esp at a car repair, thats a lot of different factors / people.

I used to work for the department and or company that does the surveys. They are probably thinking, if you had no complaints, then why not 5 stars? They sincerely use the surveys to improve their service - you gave them no way to get a better score, yet you deprive them of the top score and the prize that comes with that.

I cant answer all your questions, but i hope this gives you a better insight into why your responses are so important to them.