Ok, so when we first started in Unova, there was only all new Pokémon. Than postgame, we see things starting to change which, going by what a ranger at Route 15 says, some pokémon from other lands esacped and so they could have built new habitats in this region now.
Now in the games' continuation in Black/White 2, old gen pokémon are all over the place. I'd been thinking the same thing applied, but then I just went through the communication between Juniper and Bianca after beating Cheren. Bianca said that there are pokémon here that they couldn't prove were there two years ago. So kinda confused, why wouldn't they know if there was already towns and stuff here? Are the towns new? I thought that might be the case after talking to an npc in his house who was talking about people settling there (though he could just mean in the past), but then Pokéstar Studios. Brycen was a movie star, so surely he'd been there before. And Alder lives in Floccesy, so again, it proves it's been there.
Just a light discussion topic is all. Just seemed kinda confusing to me. Please no one be rude or anything.