Composers write pieces, and performers perform them. But for the performers, just about everything the composer writes, with the exception of the notes themselves, is a matter of interpretation. The composer indicates that a passage should be played softly? Fine. But how softly? It should get louder? Okay, but how much louder? Faster, slower? – same thing, it’s a matter of interpretation and personal taste. And tastes and interpretations can change, sometimes from one performance to the next.