I doubt a lot of people around here or the SPCA could tell the difference between a Staffordshire and Pitbull. I wouldn't be surprised if some are just mutts with the right mix to look like a Pit.
For some reason Pit is one of the most popular dogs here in the city. I wonder if the reason so many are labled that way is that it attracts people to adobt them because they know how popular they are? The most common mixes I hear about are Pit/Rott, Pit/Lab and Pitt/Rott/Chow. I think people want tougher/bigger Pits but they still want a pit. Seems like they really want a bull mastiff or cane corso.
They were actually illegal here which lead to a lot of bad breeding and people mixing pits so they can say it wasn't a pit when stopped. I don't believe they are illegal anymore.
I always liked this test http://www.pickthepit.com/