If I had to posit a theory, it would be that there's an archetypical bulldog shape; It would be something with a diamond shank, the characteristic diamond-ish profile of the bowl, with the angle in the bowl coming in at about the middle. Something close to a bulldog, but not quite, perhaps bearing a round shank or an oddly proportioned bowl, would be a rhodesian. Perhaps the difference is more like the difference between a cannon and a fugue, that is to say, it's how closely the carver follows the traditional shape.