Mary,

The accuracy of a GPS's measurement of velocity improves as speed increases. A boat ghosting along in light air will not have as accurate a reading as one flying along at 20+ knots. (Due to errors caused by atmospheric conditions, etc., you may even see a velocity readout when sitting still.)

I don't put a whole lot of faith in what the GPS shows for maximum speed since that calculation can be affected by the inherent errors. The average speed measurement is very accurate. The current speed measurement is accurate as well, as long as you don't pay attention to the occasional fluxuation.