You can’t because they didn’t. They just made it up as pitbull enthusiasts always do. On the other hand here’s a site that has numerous reports on dog bit statistics and also a link to the National Pitbull Victims Awareness collective. No other breed of dog has so many organizations dedicated to victims and no other group of animal supporters attacks victims like pitbull advocates do.

https://www.dogsbite.org/dog-bite-statistics-multi-year-fatality-report-2005-2017.php

https://www.nationalpitbullvictimawareness.org/articles/medical-studies-on-pit-bulls/

Here is a report of US and Canada dog attacks from 1982 to 2009. (PDF download) https://www.dogsbite.org/pdf/2009-dog-attack-deaths-maminings-merritt-clifton.pdf