Well, Neovo, there's no "the tune". There's the idea of the tune, which each player interprets according to their ability, the instrument and their understanding of the tune, and also, crucially, acccording to what the other musicians around them are doing. That's one reason why some sessions soar, while others seem like a bit of a struggle. A radically differently interpreted version of a tune won't be likel;y to get much encouragement from the other musicians.

I'd add that the development of tunes, and tune sets, is evolutionary- a variation (in the tune or in the sequence of a set), whether brought about by mishearing, error, the limitations of an instrument, or deliberate variation, will survive if other musicians like it, and eventually may become the accepted version of that tune or set.

A prime example of this is the migration of Calliope House from A to G. It was apparently written in A, but most sessions I've been in do it in G, perhaps because it fits whistles and flutes better. As an aside, many musicians just know it as the Calliope, which is perhaps another example of evolution.

That's tune sessions; other sorts of music, like singing, have different mechanisms. But I think the propogation mechanism is the same- it's the adoption of the variant in the community of (those who consider themselves) peers that drives the evolution.