Loud Family - Ask Scott:
"Not to say this would be true of you, but for some reason it's generally a mild shock to people that songs aren't autobiographical, when something like a film or a novel, which goes into much greater detail, is just assumed to be a complete invention."
My first thought upon reading this was that perhaps we think songs are more autobiographical because they become that way for us, the listeners. Parts of our own lives fill in the gaps in the song, the song fills in some of the gaps in our own lives. I would also say that the best novels fill the same function and are thought to be autobiographical too to a certain extent. I think films tend to be less so because there are fewer gaps that need filling so they don't become so personal. But a song, like "Inverness" which is about a feeling, a feeling we all have known at one time or another, becomes such a part of our lives when we experience it that we assume it began as a part of the composer's life too.
Just a thought.