I suspect your GPS is telling porkies Dave. There are a number of ways and variations of same on how GPS speed readouts are calculated and displayed. Number of satellites locked to, damping over time, discarding rogue positions, rounding of calculations etc.
When I had a sailing yacht, the Garmin GPS mushroom I upgraded to had regular firmware updates I installed. One allowed me user customisation to adjust speed displayed by altering damping, choosing how many decimal points, sample rate etc. as it's impossible to optimise for all speeds and conditions of use. Choosing a longer sample time gave slight delay on average speed readout but a more constant reading in rough seas. Short sample times confuse as the positon hops about due to vaguries of reception allowing momentary wild speed variations.
