What is a standard?
Not the poster. The floor.
A standard is whatever you have demonstrated you will let pass. It is established the first time something falls short and nothing happens, and it is confirmed every time after that.
Why do stated standards fail?
Because people believe behaviour over announcements, and they are right to.
A man can say quality matters every Monday. If the job that went out wrong on Thursday was invoiced without comment, the actual standard was set on Thursday. Everyone saw it. Nobody mentions it.
What does enforcing one cost?
One uncomfortable conversation, held early, while it is still small.
What avoiding it costs is a pattern, then a reputation, then the eventual loss of a man who could have been corrected eleven months earlier. That is not kindness. It is a loan taken against somebody else's future, and the interest is paid by everyone who can see it and is waiting to find out whether you will do anything.
Where do they actually come from?
From the top, and from consistency rather than intensity.
A man who is exacting on Tuesday and indifferent on Friday has not set a high standard. He has set an unpredictable one, which produces worse work than a low standard because nobody can aim at it.