Log vs GPS Speed - Why the Difference?

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If you are anchored or on a mooring the paddlewheel should indicate the rate of current or tide or the vector sum of both.
Where I have my mooring the boat is free of almost all tidal or current influences but can swing wildly. When (on the rare occasions that it is working) the boat swings around the log reads zero most of the time as one would expect, but the GPS speed can be as high as 1.0 knots as the boat swings and moves back and forwards on the mooring.
Seems to me that you need to calibrate your speed instrument, however, as others have posted, a paddle wheel log is at best an approximation and a relative speed indication.
In another life I was Second Officer on passenger and cargo ships and we had a towed patent log, with a long rope and a four bladed impeller. The clock indicator showed distance run, but we regarded the information as one of many inputs to our navigation, position finding and course setting. This was way before the introduction of GPS, Loran - we used sextants and sight reduction tables. I know I am a dinosaur!