I can't see any reason why you would need stranded ground wire other than wanting some flexibility. Skin effect is actually more pronounced with stranded wire, so that can't be the reason for it's use.
Skin Effect is the phenomenon where AC signals will flow only along the outer skin of a conductor. The higher the frequency the more pronounced the effect. This is why coax for high frequency signals can have a copper plated steel core, and microwave transmitters use a hollow waveguide instead of cables. I did a quick search, and the skin depth for 300Mhz is about 4 microns and about 3 microns for 500Mhz, so it actually is an issue at the frequencies used for 10GBase-T.
