Hmm not sure. This is what I recall from a wyze employee replying to a question either on the forums or Facebook.
PIR is just to wake the camera after its awake it looks at pixel change and based on your sensitivity setting it will decide whether or not to record an event. I don’t believe the sensitivity slider affects the PIR at all.
As for why water movement in direct sun isn’t waking the camera I’m guession that since water is good at dispersing/absorbing heat, the heat change is slow enough to go undetected.