https://banjobenclark.com/lessons/banjo-pov-pickalong-old-joe-clark-banjo-beginner
Let’s pick! The point of the Pickalong lessons is not to teach you a tabbed solo or backup, but to get you actually playing along by using your eyes and ears! Don’t forget that you can slow down the video!
It may be because Old Joe Clark uses the Mixolydian mode, which has the flat 7th tone as its 7th tone in the scale.