Hey @vea1960!
When we’re rolling banjo, we often are playing notes that aren’t in the chord, but don’t clash too hard and sound fine when rolling at a good speed. I’m guessing that the G note in the D chord you’re talking about is an open 5th string. We can’t really fret this easily to make it be a note in the D chord, but we still want to play it to keep the roll pattern steadily going. Often, we find that that’s more important than playing all the “correct” notes.
The C note you’re referring to makes the D chord a D7 chord, actually. This adds to the tension of the D chord and makes it want to resolve back to G stronger. It’s basically a way to modify your chord to make it point somewhere even more eagerly!
Also, which version of Boil Them Cabbage Down are you referring to? I can’t seem to find one on the site where the G and C chords both start with their root notes. In the normal (index lead) version, the G chord starts on a B note.