Probably just boils down to drivers. I've seen similar benchmarks.. on my hardware anyway (AMD), gaming has been consistently more or less identical to 7, and when there is a difference its pretty minor at best, plus or minus a couple FPS. Where I do notice a nice improvement though is load times, be it booting the OS or just applications in general, just seems a bit quicker there across the board especially when it comes to large applications like Visual Studio 2012 and whatnot. A cold start of a fully loaded VS2012 install with big addons like CodeRush and the like starts up a fair bit faster under 8 than 7.
I was readed some good benchmarks of windows 8 vs windows 7 gaming performance, but in my pc is not the same story.