Why are violins harder to tell apart than human voices?

“Players and makers often talk about differences in tonal character between Stradivari and Guarneri del Gesù violins,” writes violin maker Joseph Curtin in the November-December issue of Strings magazine. “Numerous listening tests have shown that even expert listeners have trouble telling nominally very different violins apart.… I would rather consider a more general question: What kinds of sounds are humans good at recognizing and comparing—and do violins make these kinds of sounds? … Not only do we recognize the voices of people we know, we also recognize general qualities in the voices of strangers—an Irish accent, for example, or the effects of a common cold.… If there is indeed a distinct Old Italian sound, why isn’t it as immediately recognizable as, for example, an Italian accent? An obvious way in which the voice differs from the violin is that ‘player’ and ‘instrument’ are one and the same…. The recognizable attributes of violin sound are more related to how the player uses the instrument than to the instrument’s innate tonal characteristics…. Just as the ear adapts itself to the acoustical environment, violinists adapt themselves to individual instruments. Great players find ways to project their own distinctive voice using almost any violin.”