It's not solely dependant on upbringing. Genetics will play a big role. If they're from a fighting background, they will be more likely to be aggressive (especially dog-aggressive) than pit bulls from a different background - say, show line AmStaffs - or other breeds.
And while it's true that dog-fighting breeds were traditionally not bred for human-aggression, in practical terms, a large and powerful dog that is strongly dog-aggressive is more likely to end up biting a human than a dog who is not dog-aggressive. An awful lot of serious dog bites and maulings seem to stem from dog-on-dog attacks. The attacked dog's owner intercedes to save their dog from severe injury or death, and the attacker starts biting the human too.