The article on pitch in the Grove Dictionary of Music and Musicians is quite revealing: It seems that there was no such thing as standard pitch until well past the Baroque era. Voices sang where they were centered, the way I like to have my vocal tuning classes work. Introducing fixed pitch became the practice as instruments were added in to fill out the sound. Even then there were deviations from a given pitch.

It would be good, I think, instead of arguing about whether A should be 415 or 440 to postulate A's of all different frequencies. Incidentally, that's why a lot of Renaissance vocal music is available in modern editions in a wide variety of keys, some of which make no sense at all to me, like F# major. I have nothing against F# major but it is not a tonality in which people ordinarily sing, whereas F is, as is G.