My G21 fouled primarily in the last inch or so of the muzzle end, shooting factory Blazer Brass and UMC. As for the rifling, I'm thinking it's really hard to see it clearly until it has been cleaned. If you count the indentations at the crown, you can see there would be 8, evenly spaced, identical grooves, as per usual.
Edit: in fact, now that I'm looking carefully, what I see is the fouling is building up in the corners, where the lands meet the bottoms. It is building up heavier on one side, and thinner on the other. There would be 16 strips of fouling, total. 8 thick, and 8 thin. There will also be 16 dark/clean strips which look roughly the same size, and come in pairs. The similarity in width is deceiving. One is actually the top/side of the land. The dark strip immediately CCW to it (if you are looking down the muzzle) is the clean portion of the bottom. Match things up with the indents in the crown, and you'll see what I'm seeing.