Seems to me that the pressure monitor on the bike is more of a fancy warning light than a accurate reading of the "tire pressure" as we have all come to understand. I was always taught to check the tire pressure when the tires were "cold" meaning before you began to ride. And the TPMS on the bike gives a warning signal only if it reads a figure plus or minus 5 psi I think.
I don't use the TPMS to determine accuracy of the tire pressure. I use a gauge before I ride.