The problem with steam is their philosophy is "Screw the Players!"
They code what they want, when they want, as POORLY as they want, and for the highest end PCs they can get their hands on. They have almost never made anything that runs decently on mid-to-low end hardware, and just you TRY running HL2 on minimum settings on your HL1 rig. It looks like dog poop, and is about as smooth.
So, by definition, their games have high requirements. Thus, by definition, low-end players do NOT use steam, which cuts out a large portion of the gaming community in that hardware poll they take.
So it's skewed (biased) towards the higher end of the spectrum.