I recently went down hole of comparing the strava app to my 510. Water Co and Nesh have a thick tree canopy AND bodies of water that just hold the humid in and it is usually the highest when I ride in the AM. Anyways, I noticed a few people getting PR's / crowns with the app and their GPS line was all over the place. Anyways, the comparison was pointless because there was no control to compare against. But it just proves that if you have really twisty trails and heavy tree canopy that you are going to see some funky results. And the faster you go, the worst it gets. As noted, this is less of an issue on the road but can still happen for the same reason. So really if you are within a sec or two of the KOM, it is basically the same time since there is no way to compare the accuracy between the two times.
All that being said, I am 1 sec back on two fast sections currently and it is annoying lol