I was going to suggest it could be dual-socket, with HyperThreading enabled, but looks like the survey is HyperThreading aware and only counts real cores.
Unless I'm misremembering, the first dual-core CPU on the market was the Athlon 64 X2 in 2005, so I agree that these results are likely from servers running Steam.
Edit: a quick bit of google tells me both Intel and AMD released dual core CPUs in May 2005, with Intel beating AMD by a few weeks, so there you go.
So must be Quad Socket motherboards.