Ah.
I haven't seen many DX9L Vista SP1 benchmarks vs DX9C XP SP2/3, but a big part of the reason DX10 is slower than DX9 is because they redid the architecture of direct3d and completely removed the cpus handling of it. Now its almost completely handled by the gpu without getting assistance from the cpu.
Other than that it would of course be because its handling more complex graphics.