I must say that I have never heard that a crossbow would be more accurate than a musket as much as it was just easier to handle, cheaper, had quicker reload time and less sensitive (no fire nor dry powder to maintain).
Edit: The only time the accuracy comes up in the sources I have read is when compared to bows for untrained or not-as-trained people. Basically a bow is a superior weapon, if you spent your childhood training for it. If you haven't, you can learn how to kill both prey and enemies after a 1 week crash course with a crossbow.