The original federal mandate for TPMS is that it shows "low" when pressure drops below 25psi, so a lot of companies will use sensors that only have 2 states "OK" or "Low" rather than actually sensing the exact PSI and reporting it back (I'm guessing cost reasons).
Nevermind the fact that 25psi isn't the right place to indicate "low" for every type of tire/vehicle, but that's what happens when you get legislation involved. They did end up revising it to say "within 25% of the manufacturers recommended pressure", but still a lot of manufacturers stuck with the old 2 state sensors because they're cheaper, and again, 25% low isn't necessarily the best "low" point for all vehicles. The net result is that the system is almost worthless on some vehicles (for me, specifically for my sports car with sport tires, because even 10% low is VERY low) and you still have to check it manually to actually get precise results.