If your watch is like my garmin, then it records data points at a constant rate (ie one point every second). So the faster you go, the fewer data points are recorded and the fewer times you are adding error to your distance....
Looked at the distance on google earth and it looks like 410 metres. Faster was more accurate but still a bit wrong by the looks of it.
A few laps on a track would be a good test as well....