One potential reason could be that the GPS is not fully locked on your position when you start the race.
Once you start the "Run activity", make sure that the GPS is fully locked before starting the run. You can see it on the watch face, starting red then finally full green. I'm near high rise buiding and sometime have to wait for 30sec to 1min for the GPS to be locked.
To get the best GPS location, check that "Settings" / "System" / "Advanced" / "Satellites"is at least on "All systems" or "All+Multi-Band" (I use All Sytems and found it reliable enough)
