Despite what Paizo Claims, Golarion...though a single setting...is a single setting much like Spelljammer or Planescape is a single setting when you include their entire multiverse (Dragonlance, Forgotten Realms, Darksun, Greyhawk) inside of them.
Golarion is composed of many different areas representing a multitude of different campaign settings. Want that Techno/Fantasy campaign setting...they have it. Want that Pirate Campaign Setting...they have it. Want that Asian Themed setting...they actually have one of those as well.
It's one world, but basically a ton of different campaign settings set inside of it.