It's a little confusing how you display the accuracy for each device. The Polar chest strap was the most accurate as you mentioned, yet your table shows 0bpm difference for the Mio, and the Apple watch, and 1bpm difference for the Polar.
How did you collect the data for each device? Did you record the heart rate for the whole trial with each device watch to come up with that bpm difference?
It would be interesting to read a bit more in depth summary of how you tested because many other articles had different results than you seemed to have gotten. I guess, when you're trying to show how accurate something is, it's helpful to be as descriptive as possible because I'm more confused on where to go for a wearable now... it seemed like this was very open ended and subjective so I'm not sure if this was your experience or what I can expect also.
For the stress test, I wore all of the devices listed in the article at the same time, while walking/running on the treadmill. I recorded the results for the devices on my left arm first (the Apple Watch and the Mio), and found that they were both off from what the EKC machine was saying. I tightened the Apple Watch and the Mio, and then took another recording of the results. This time, they were both accurate in comparison to the EKG machine.
Then I did the same thing with the devices on my right arm and the devices I was wearing across my chest and in my ears. Overall, the Polar H7 was the most consistently accurate because I never had to readjust it like I did with the Apple Watch or any other wristband.
Keep in mind, I had to wear all the wristsbands very tightly around my wrist. Personally, I would never wear any watch, smartwatch or wristband this tight on a daily basis - only when I wanted to accurately track my heart rate. I hope this helps clarify things!