We're those dogs DNA tested?
Here's a good article about how statistics can drastically change when you use umbrella terms like "pitbull" to speak about a very large group of dogs, then use the term "pitbull" in a literal sense when measuring the amount of actual pitbulls in the United States. It's easy to manipulate.
http://blog.hugabull.com/a-closer-look-at-bite-numbers/
I just saw you posted a dogsbite.org website after writing this. It's been a well known statistic manipulator and has been condemned by the entire American veterinary community. It's like sharing information about gay marriage from conservativetribune.com
The only statistics that matter are those related to dog bites from the CDC or veterinary association as they have no hidden agenda in their numbers.