it's like the difference between mathematics and philosophy-

tunes are 'math', and they need to be right...songs are philosophy, and vary more and differences are tolerated, in many versions and with a story of image...so it's all just a different mind-set.

I am awed by the talent at a good tune session...but I quickly get bored, as I can't play that stuff, and you 'almost' need to play to appreciate the flow for any length of time...with songs, even accompanied songs, it is easier to just sit & listen