if you are in an orchestra or a chamber group or playing a solo piece of classical, a piano sonata for example, you will not go down well if you piss around with the tune (as in diddley), add capricious ornamentation here and there (as in diddley) and change the rhythm (as in diddley - subtly) intended by the composer.

And how much variation did you ever get between different run-throughs of the same set with a band like Planxty?

You aren't comparing the same performance situations.

Listen to the beginning of each of these versions of the Bach Violin Partita #1:

Arthur Grumiaux

Jascha Heifetz

Rachel Podger

Gidon Kremer

Alistair Brown (viola)

Henryk Szeryng

You probably do get occasional performances of Irish tunes that stand out as being as far from the norm as the odd one out in that lot, but I doubt you ever get such determined effort put into being different from everybody else. (In Scottish terms, I would say the player I'm thinking of is doing a J.F. Dickie).