Gps-speedsurfing

Hi

I've just started speedsurfing with my GW-60. After my first registration &500 shown on the GW-60 (the watch) is 6.11 kt, gps-speedsurfing.com ranking shows &500 16,77 knots and when I see the &500 under my sail it shows 10.25 knots. Why isn't &500 the same on these 3 places?

Thanks

NEILPRYDE