Hi all… Quick question on ground speed: What does this actually measure?

I have tried to calculate the average ground speed of a flight by taking the time between departure/arrival runway actual times and adding up the distance between all the position data delivered by GetFlightTrack. Did a sense check by comparing this to the great circle distance for the flight - so the average ground speed I calculated should be correct.

However when I search GetFlighTrack feed for the highest recorded ground speed this turns out to be a little less than the average I calculated - which clearly is impossible. So I’m wondering if maybe your definition of ground speed is different from what I expected?