Too bad they don't track by breed
Though this is an important metric. I don't think pitbulls are inherently more likely to bite, but I do think they cause a lot more damage when they do bite. Minor bites are unlikely to always be reported. Note that all of the dogs listed are big powerful jawed dogs.
It's very plausible that a responsible dog owner would have a dog out in the fenced yard when a postal worker came by, and then the breed is definitely going to be a factor.
Most people saying this have no proof of their claim.
(I've been bitten twice when riding, both times by labs. They're goofy dogs, but have a mean streak if they don't know you.)