There was a time when PROBE was generally off by a full 10°c but that was back in the KT133A era, I haven't heard anything about serious temp problems in the KT600 systems. But any system where you're using something in the motherboard, instead of inside the chip (or touching the side of the core itself) is bound to give readings that aren't 100% accurate, I recommend going by what Ffats said and when confronted by two different of numbers erring on the side of caution.