Connected GPS: Distance calculation bug(?)

I did few runs with my Vivomove Style lately (after 4.70 firmware update) and I have noticed that summary distance shown in Garmin Connect seems to be overstated despite a very smooth GPS track.

After extracting all GPS datapoints from the track file and calculating distance manually it does look like the watch calculation algorithm has errors.

This seems to be consistent across multiple activities:

Activity 1: GC reported distance 5.01 km, distance calculated from GPX 4.75 km (-6%)
Activity 2: GC reported distance 5.01 km, distance calculated from GPX 4.80 km (-4%)
Activity 3: GC reported distance 9.01 km, distance calculated from GPX 8.71 km (-3%) 

Had anyone noticed this? Is it possible that while the track comes from Connected GPS, the distance totals are based on accelerometer (non GPS) data?