My music teacher recently pointed out to me that, when performing a trill (alternating between two notes very quickly) the finger for the higher note should be placed slightly lower on the string than it would be when played individually. In other words, when performing a trill the frequency corresponding to the usual position of that note is ever so slightly higher than it should be. I know that stringed instruments like the Violin, Viola and Cello are not "well-tempered", which leads to many such subtle changes in frequency when it comes to double-stops, trills etc. But is there a clear, physical explanation for this phenomena? Or is it an auditory illusion?