The GM oil life system is explained at length in this thread:
The best quote from that is probably:
"The GM engine oil life monitor counts engine revolutions and accumulates the number for the basis of the oil life calculation. It then adds deterioration factors for operating temperature, start up temperature, soak times, ambient, coolant temperature, etc... There are a LOT of factors that "adjust" or affect the slope of the deterioration but the fundamental deterioration is traced back to the ZDP depletion that is inescapable with engine revolutions. The specific rate of ZDP depletion is readily measurable for any given engine so that is the fundamental item that is first calibrated for the oil life algorithm to tailor it specifically to that engine."
Remember, this system does NOT monitor your oil. It just runs what amounts to a computer simulation of the worst case for your oil's rate of failure based on your engine's usage as perceived by the engine computer, including throttle settings, RPM and gobs of other factors.