A breed can contain multiple dogs, here is a Wikipedia definition -
Pit bull is an umbrella term for several types of dog believed to have descended from bull and terriers. In the United States, the term is usually considered to include the American Pit Bull Terrier, American Staffordshire Terrier, American Bully, Staffordshire Bull Terrier, and sometimes the American Bulldog, along with any crossbred dog that shares certain physical characteristics with these breeds.
People just assume breed means a singular type of dog; it doesn’t. This applies to all breeds of dogs (Retrievers and Shepards for example). There are over 300 breeds and this one causes more deaths than the rest combined.