Where does the difference between calculated and actual groundspeed come from?
I have a little paragliding experience. While only a tiny fraction of yours, I think I can answer your question, which boils down to: data accuracy.

The trim speed of a fixed-wing aircraft would depend on its weight. Without accurately measuring the pilot's weight (FYI the measurement in small aircrafts is precise to pounds / kilograms) and plotting that against a chart, the speed you were told in school is only an approximation.

I (like others in the comments also) question the accuracy of your speed data.


To accurately measure forward airspeed, you'd need a pitot tube pointing in the right direction.
Or you can use a wind measuring device which measures speed and direction
To measure your ground speed accurately you'd need a GPS.
To combine the two (wind speed + ground speed), you'd need some sort of computer to do the computation, either in real time or with recorded data.
The ground station only measure wind speed at ground level. When you're up in the air (albeit relatively low compared to aircraft), the wind can be different.