Easy (Score 1)

This is easy to explain: 1. 99% of all CS hacks out there require openGL. Apparently it was much easier for them to hack openGL than D3D. 2. The D3D renderer in HL1/CS sucks. It used to be full of bugs. I think they have fixed most of those in the last year or so, but Valve never did optimize it enough to keep up with their openGL renderer so the fps is lower in D3D.