Pitbull by rural/gross definition is any dog used for fighting in pitts. Thats what you're failing to understand, it's not a "breed". Staffordshire is the predominant breed in "pitbulls" because of it's genetic dominance and recognizable phenotypes (wide mouth, pronounces occipital orbae, etc...)

90% of the dogs you buy as purebred pitts are truly mixes, thats my point. Just like purebred labradoodles.