They couldn't before, but now with everything consolidated into Direct X it's a lot easier to arrange lines of titles into a "generation."
@II-zOoLoGy-II
You have to think of it differently.
Think of devkits for next-gen consoles. They've been out for some time now, but the consoles themselves won't be out till at least fall 2013.
Just like how DX11 has been around since 2009, that allowed developers to start working with the API in their engine building, but like I said before, most games were continued to be built on DX9 as a base, with DX10 and 11 features layered on top. This was a way for game makers to keep their leading platforms to be consoles, then porting a slightly enhanced PC version. Keeping to DX9 was also cheaper for certain PC-only developers or to sell better since not many people had DX11 graphics cards of Windows 7 at that point in time.
Now, way more people do have the capability and the companies releasing PC only titles in the near future building up new engines have the opportunity to create them as DX10 games from the ground up.
Current-gen PC games are built on DX9 first, next-gen PC games are built on DX10 first. Not sure why you're so caught up on me saying it was a next-gen title, it's just a fact.