Comment by heikkilevanto

Cataloguing classical music has always been a headache, even for experienced librarians. There are multiple recordings, some conductors record the same piece multiple times, with same or different orchestras and soloists, arrangements for different instruments (maybe in a different key), parts of a piece known under alternative names (Bach's Air for a G-string is just one movement from his orchestral suite, and he never called it that anyway). Old composers often borrowed bits or rearranged whole pieces (Mozart did his version of Handel's Messiah), and various historical ways to name and number pieces (concerto #1 in B from opus 6).