At a guess if thats what it says then thats what it means. Error messages don't come much clearer. You are trying to run a DX8 Benchmark running DX9 - this could (and probably would) give a false reading as DX9 is faster than DX8. So to stop that they probably make sure you run DX8. Simalar to if you where running DX7 then you would get a low reading.