Me neither. But it makes sense. In some countries there are a lot of OS X users.
Just like there are a lot of iPhone users. Generally they are users that get a Mac the same way they get an iPhone. It’s from Apple. It looks like premium hardware. And it seems simpler to use than the alternative (Windows).
They don’t get the Mac for gaming. They don’t care about the GPU at all. But considering there are many users if just a few of them decide to try the MacBook to play some light game they can put a good number in the steam survey.