When I was running on treadmill the distance was measured really good up to 11th km, based on steps. After reaching 11k the watch showed the time of the lap 11 and the distance counter reset to 10. Then, when I got to 12k, time of lap 12 was shown but distance counter was set to 10k again. And so on with lap 13, 14... I finished the run with 15 laps, 1 km each, but total distance was 10 km and this value was used to calculate average pace.