To me and seemily people my age getting an 'A' or 5/5 means "no problems" not "exceptional".
You don't express derivation in the positive direction because delivering a product or service as described without issue is the highest aspiration. It feels wrong to make anyone dance for their supper.
It's not about making anyone dance - it's about rewarding people for exceptional service. Sometimes, when I pay someone to do some kind of service for me, they go out of their way to be better than average. That is just what happens in human-to-human interactions, and I want to honor that. It's the reason to give a bigger tip than normal (now don't tell me you never did that and always tip whatever is your absolute maximum), and it should also be the reason to give more than the average amount of stars.
Oh, and btw: it's the same the other way round. I also sometimes put much more effort than strictly required into my job for a particular client, and I love it when that is recognized and honored. If it is, I am more inclined to do it again later. If not...well, not so much, then it's probably back to "standard service, just enough to fulfill the contract".
Imagine you are making barely above minimum wage and your entire income depends on your ability to constantly make customers feel like they were serviced "better than average". Sounds incredibly exhausting.
I mean in a general life sense I completely agree but not when were talking about grading others. Setting the bar for perfection at "exceeds my personal ever-changing expectations by an arbitrary unspecified degree" would actually cause me distress to have to carry out. I could never judge another human being in such a way and I would feel, and have felt, degraded being judged that way.
Like why even set expectations in the first place if your bar for perfect is "impress me"? Why is "I did everything you asked" less than perfect?
Who's talking about perfection? Perfection is like zen -- it's a goal that can never actually be achieved, so there's no problem in not achieving it. Striving for it nonetheless is a good thing.
You don't express derivation in the positive direction because delivering a product or service as described without issue is the highest aspiration. It feels wrong to make anyone dance for their supper.