i say a good grade hand held tire gauge.
there is such in my pick up and it is a fail.
i set the psi ( with my gauges) and the readout gives varied numbers.
then if you move the tires around. or get new tires did the tire peps put them back as how the disply is set up?
batteries fail.
sensors fail, or just give false readings.
time of day,
outside temps,
alt barometric psi.
all can cause differnt readings. then add in heat. type of road did you just do a hard push in death valley, etc.