I'm guessing the ABS sensor, but not because of distance traveled. The amount of brake surface area has changed, so has your overall stopped power. The pressure you are able to apply to the brakes is now concentrated on a smaller area, thereby creating more PSI on that smaller braking surface instead of being distributed over a large area.
I'm not sure what the ABS sensor measures exactly. I'm guessing there is a longitudenal motion sensor, that tells the computer that you are still in a forward/backward moving motion, and that your stopped wheel shouldn't be stopped. The computer then releases break pressure (pulsating break motion) and re-applies it as needed. However, I'm not convinced that it actually measures the PSI in the line or any combination thereof.
Someone who knows ABS systems will need to chime in. I'd be interested to know precisely WHAT the ABS system measures, other than forward/backward motion.