Wouldn't all musicians who are good enough to work for a professional orchestra be able to play all of these difficult pieces without making any mistakes? Being able to play difficult pieces without making a mistake is the objective definition of perfection. Yet among the people who can all play difficult pieces perfectly, not all of them can work for the New York Philharmonic or more high prestige group. What differentiates one player who plays perfectly from another player who also plays perfectly?
There must be something else beyond 'not making mistakes'; that thing is subjective, not objective. It's pretty clear if someone hits the wrong note, but who gets to judge that a particular musician held a specific note for 10 milliseconds too long? Or who is to say that some notes were given 'too much emphasis'?
I'm sure that there is consensus among the elite musicians about all these subjective judgments, but it doesn't change the fact that it's subjective... The audience might not care at all either way or they might even disagree with the elite musicians' consensus... Who is right in this case? Isn't the customer always right? What does it mean to have a 'trained ear'? Can't the ear be over-trained or mis-trained.
If you teach someone how to do something the wrong way (e.g. inefficiently) and they learn how to do it the wrong way perfectly, isn't it still the wrong way?