The important thing is with the number of Steam Users each 1% is more than 1 million people. Thus < 50% can still be a lot of people. (4.39% own a 970 GTX GPU, 0.92% own a 980 GTX GPU, 0.72% own a GTX 980 Ti ~= 7.5 million people who don't need to upgrade graphics card, now add ATI and SLI.)

PS: Active Steam account = bought something from steam AND logged on in the last 2 weeks. Which IMO is a much higher hurdle than most website tracking. And clearly not all gaming PC's are on Steam.