Clarification of nicknames.
Am Staff = American Staffordshire Terrier or the AKC registered version of the American Pitbull Terrier. Thick and blocky; seem like dullards to me.
Staffie = Staffordshire Bull Terrier, the smaller dog spun off from with GB fighting dogs. Small and cobby; but rascals and plucky as hell.
Pitbull = American Pitbull Terrier or what you have. Typically leaner and more athletic than the Am Staff (there's more diversity...