If customer is always right, does it mean service provider is always wrong, even if they have been trained and well prepared for the job?

No, its just that some of the  customer aren't contented with the services or maybe they havent meet with their satisfaction.

No. What we mean about customers being right is that we as service providers will do all that we can to make them satisfied. But in the instance that its beyond our limitations, we just have to make them understand and replace it with something that we could offer them to still make them satisfied. With that, even with our limitations, we are not wrong.