Why is the trip odometer changing even though I am sitting still inside?
Errors in your position fix make the unit appear to be moving. Since you're inside, these errors could be substantial and can add up to a lot of distance.
Zoom in as close as you can to the track and you'll see a "spiderweb". Add up the legs of the spider web, 5 feet here, 5 feet there, and you'll get large error Boyd describes.