vivomove Sport heart rate accuracy

I did a stress ECG earlier today; this is where you are hooked up to an ECG machine while running on a treadmill, and the speed / incline increases every 3 minutes.

Every time I checked the heart rate on my vivomove sport it was under reporting compared to the ECG machine. In the earlier segment it was 20 bpm off. At the end, it was reporting 113 but the ECG machine had me at over 170 bpm.

I thought my watch was loose, so I tightened it early on, but that made no difference.

Is there a way to improve heart rate accuracy?