Hi MRB.
Sounds like you know more than me, I havent heard of the "pet safe" sensors, then I havent played with PIR detectors for many years.
Are they definitely PIR & not ultrasonic.
As pets are generally smaller than us hyoomans (unless you have a pet cow or horse inside) then the IR emission area will be smaller relative to the sensor coverage area. So maybe the design of the fresnel lenses is significant ?, but then size is relavent to distance from the sensor or maybe the intensity of IR ?.
Humans have a lower body temperature than dogs & cats, maybe the sensor has temperature sensing as well. but then insulation(fur) variations will probably affect this measurement.
All speculation on my part, maybe someone else has some ideas or actually knows how these specific type work.
There is a fully automated system in australia that uses video & a laptop with image processing to distinguish different animal types to help manage feral & stock control.
The computer controls a gate that directs animals into or out of an area.
You could try calling some security system manufacturers.
Tonig.au