I had that problem as well... On top of that, (sorry to threadjack) but when I cleared my personal best and ran again on the treadmill, I ran btw 12km/h and 14km/h for 45 mins and and the end of the run it says my best 5k was 26'51" (which is close to correct). but it said my best kilometer was 6'10" (which is closer to 10km/h and a pace of ~30 mins over 5km).
So how can it say my best 5km be 26'51" (average of 5'20" over 5km) but yet my best km is only 6'10", especially when I didn't even drop below 12km/h?