Why does cpu do better job during the light cache step than gpu?

[zach_ryder]

why does the light cache process always look better and more effective than the gpu light building process? the gpu light cache in this example was set to Subdivs: 5000 and retrace: 5.

cpu Subdivs: 1000 and retrace: 2

Why such a drastic difference? Should I double the gpu light cache subdivs to 10000?