I recently started using one of these. The other day, I was working out on a rowing machine and received an alert (twice!) that my heart rate was extremely high. I was surprised because I wasn't even pushing myself that hard, and I am a fit person in general.
Upon research and asking around, I understood that it's more of a false positive. A more paranoid person may have reacted differently in such a scenario. So, consistency alone may not always suffice.
You need the band on your tracker to be very snug or this is likely to happen. You're actually seeing the tracker move, not the heart rate.
I've seen substantial mismatches between wrist and chest strap if my tracker had any ability to wiggle. With an infinitely adjustable metal band set so there's no slack but no pressure I see a good correspondence, although I haven't put it a test as hard as rowing (my activity of choice is hiking.) But my wife absolutely doesn't like such a band, wearing a soft, loose band and using her arms her "heart rate" has gone above 200.
I have also seen an environmental effect--these days I use the chest strap sensor because if my wrist is cold (conditions just below when I would add another layer) the wrist data can go totally bonkers.
Upon research and asking around, I understood that it's more of a false positive. A more paranoid person may have reacted differently in such a scenario. So, consistency alone may not always suffice.