Bryan ,you have missed his point. fiddles are designed[and here I must correct Ian]so that they can beplayed in   equal and mean /just temperament,mandolins and other fret can only be played in Equal Temperament.
Ian,when a fiddle player plays with a concertina or a mandolin he is forced to play in Equal temperament,when he plays on his own,or with an instrument in mean temperament ,he can adjust and play differently.