It works from the ABS wheel speed sensors, rather than monitoring the actual PSI in each tyre.
It checks for differential readings from the wheel speed sensors, but only after carrying out initial self-calibration checks each time you start and drive the vehicle.
Maybe you confused it, with not one, but four different readings from the previous tyre pressure "set", and it was constantly re-trying to calibrate the ABS, but eventually gave up, eventually throwing the warning light. It would have probably reacted much quicker if just one wheel gave a change from the stored parameters, as you usually (unless you are extremely unlucky) just get a puncture on only one wheel.
Differences in rolling radiusses (radii sp?) from tyre wear can also affect the monitoring system, but these would be not noticeable under normal use, only triggering the TPMS when a tyre is changed!
HTH, rgds
