Instrumental vs. Choral Musicians
In addition to being an organist, Isaac is a choral conductor. I am fundamentally an instrumental musician ('cello), although I do some singing, too. Frequently I complain to Isaac about singers who seem to have no rhythmic sense and, in particular, no sense of rhythmic pulse, the feeling that there is an uninterruptable flow to the beat of a piece of music. It's been my observation, that I've grossly generalized, that vocal musicians have no hesitation about putting bits of extra beats–even one or two entire beats–into a bar of music. This is the sort of thing that makes instrumentalist (and dancers!) pull their hair out, or break their legs.
So, Isaac and I were talking about this phenomenon on the way home tonight from dinner, and this time is when we made the connection between the instrumentalists and the dancers contrasted with the singers: the singers are not very physical at all when they produce their music, they just move their mouths, whereas instrumentalists (particularly string players) and dancers are physical when they produce (or respond to) their music. This stuck us as a very sensible guess at what might be at the root of the extra-beat phenomenon.