Which device are baselining against ? One is 72 and one is 53. Which one is the more accurate?
Perhaps your actual HR is around 60 and each device is around 10 bpm either way ?
It is hard to know which device has the better accuracy. How do these devices read if you were under exercise, jogging or walking? Are they similar? Often it is the resting heart rate that shows different but 'under stress' they are closer in their readings.
There are a number of unknowns here, and although you assume the watch ultra is the one that is out of tolerance, it may be they both have a minor inaccuracy. It may be the watch ultra with updated sensors is more accurate than the other device.
Only if you benchmark a number of different devices can you tell which is actually showing the error.
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