I think it evolves over time as well.
When you first start writing a song, you tend to improvise a lot and come up with ideas. As you keep going you start to narrow down the best parts and eliminate the bad ones.
Then when you start recording, it takes a whole new direction again because the other instruments dictate what the song will sound like.
When you get a rough draft recording down, you start to realize that the free form stuff might now be working, so you start editing it down to a more structured approach.
Then you move on to doing final takes and things take a new direction again because you start adding lyrics and changing melodies.
Then you realize your song is still over 5 minutes in length so you have to shave off that cool solo in the middle, or cut out 4 bars of the intro, or fade it out earlier...
By the time you've spent a month on it, what started out as an original sounding freeform idea has totally changed into a standard pop song.