Not all instruments are limited to a fixed set of pitches. A good classical string player micro-adjusts each individual note to adapt to its harmonic context. For example, making all the thirds and fifths sound good even when the key changes, or adjusting a leading tone up or down very slightly to make it even more leading.

Another way to think of it is that they have to hit every pitch without assistance from the instrument anyway, so they learn to make every note sound “good” rather than hitting a mathematically defined frequency.