Hi I've tested the Leikr on 3 runs. Every run is about 15% longer then the measured distance on Endomondo.
You can see my run below, it is obvious that the zig-zag is the big problem.
At about 1,7 km, I switched to map view - you can see on the track that it makes a bigger zig-zag just then. The same happened just before and after 3 km, when I tried to get the zoom right - It goes of track every time it has to "think harder" when switching view.
http://www.endomondo.com/workouts/274859599/288959
Hope this can help you - otherwise, fantastic watch - have recommended it for all my running friends i DK and Norway.
/Rune