Not everyone uploads specs...
I have a 5Ghz 3770k, a 1.4Ghz R290 with 8Gb 2400Mhz RAM and I've never shared my information with them, loads of gamers don't.
As it stands currently there's more PC's that are more powerful then PS4 according to the survey right now but that's without factoring in people like me who don't share the hardware information.
Then there's the FACT that they don't send the survey out to everyone, they pick random machines and then work out an ESTIMATED percentage based on that.
So there's a lot PC's out there that can whip the consoles asses then consolites like to admit.