On one particular vehicle I drove for work with an outside temp gauge, the temps when sitting still were quite variable compared to the "real" outside temp.. Once moving for a while the readings stabilized close to "reality".
Keeping track of just when it reads "funny", you may learn over time what effects it the most. Sun effects have been mentioned, icing can actually "freeze" it at 32 if it gets caked with enough ice to keep it's temp stable, warm air from the engine area would bump up the readings at low speeds or when stopped (even though they TRIED to isolate the sensor it still gets effected)....
Just my personal observations after using that factory model a few years, followed by mounting an aftermarket unit on another rig that showed similar reactions.
My current rig has one that sometimes gives useful info., but I gotta tell ya - the educated eye for snow & ice conditions is still what I have to rely on. Ice Warning gauges just sound off at temps near freezing, & really "cry wolf" quite a bit - too many other factors determine when the road gets slick.....