Meetup countdown not equal to distance covered

Over the weekend I set myself a 600km target via a meetup event - 15 laps of Out & Back Again. I noticed that each lap, the distance covered went up by 40km on the nose, but the countdown went down by 39.9km, so I had another 1.5km to do at the end.

Probably not all that significant for most rides, but thought I’d flag it in case it’s connected to anything else!