Hey Noel great tip, you've hit the nail on the head here. Normally Band-in-a-Box will look in its source material for RealTracks for a clip that matches your chord exactly (if you have a C it will look for a recording of a C etc.) With that box checked Band-in-a-Box is allowed to look for audio that is close but not exactly on the chord, so it might find a phrase that was recorded in B and then shift the pitch to C to match your chord. This allows you to have a bit more variety in the generated tracks, but if you're already stretching the tracks past their default tempo you'll get artifacts for sure.