Actually I think that GUEST put it pretty well about how a writer will let a song go "as a gift to all the listeners, knowing it will change subtly for each one change subtly for each one."

Some people find that hard to accept, and think that a song can only mean precisely what a writer had in mind when they wrote it.

As often as not, myself, I only understand what I had in mind with a song I've written after I've sung it a good few times, and ideally heard someone else sing it.