Coros vs Garmin heart rate straps. Which one to trust?

I previously tested the Coros optical arm heart rate sensor against the Garmin Run chest strap.
Recently I've suspected the Coros strap was giving false readings so I compared the two again with some interesting results.
NOTE - I am testing the Coros Optical arm band and the Garmin Run chest strap NOT the optical sensors on the back of the watches!