The Natural Philosopher wrote:

we don't disagree that the rate changes. Only by how much

As there was nothing on telly tonight, I used my OBD cable to log the
four wheel speed sensors over a straight section of road with cruise
control set to 50mph, which was measured by VCDS as 77.6kph then let
some pressure out of the right rear, turned around, rinse and repeat
until the TPMS triggered.

Starting at 39psi the right rear was on average 0.1kph slower than the
other three wheels, it triggered after letting it down to 31psi by which
time it was on average 0.2kph faster than the other three wheels.

So it triggered on a increased wheel speed of 0.3/77.6 = 0.4%

and why.

The numbers don't help with that.