I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'd have to say that it would only take one or two sizes to be "right on" with jetting if it's at all out of tune.
It surely doesn't account for the difference between a 500 and 620 main jet.
Try putting a 620 main jet in a carb that was tuned for a 500 main jet and it'll be ridiculously rich, regardless of what type of riding you're intending to do.