HR Accuracy of Vivosmart 4

(sorry for my broken English as it is not my mother tongue)

I bought the Vivosmart 4 yesterday and updated S/W 2.60.

Tested its HR accuracy of Vivosmart 4 by comparing with the combination of Garmin 935+Scosche Rhythm 24. Workout; 45 mins indoor cycling.

In the attached pictures show the HR reading from Scosche (picture on the left) was quite a reasonable while the Vivosmart 4 (picture on the right) shown quite a low HR (below 80 bmp) at the beginning, then rose up to a more a more reasonable reading from the final 10 mins of the workout!!

Is it something wrong with my Vivo4 or does it need a few days to warm up?

Thanks a lot.