I expect the sensor is located in front of the radiator (it was on my outback and is on my wrx). Perhaps it's been hit/moved closer to the radiator and therefore is reading that heat.
My temp display will read high if I'm sitting in traffic or idling somewhere else, once the car starts moving it will drop back down to "Actual" ambient temp (because it's close to the rad)
If that's not it, I wonder if the sensor could be failing? Maybe a small short in the wiring?