That could be caused by sun shine, if its not mounted in a shaded area, or on the North side of your home. Can also happen if its near a sofit vent.
Not unusual for an air port to have one temp, and a mile or 2 a way have another temp, a couple degrees cooler or warmer.
But jumping that much. Sounds a bit out of line. Does it seem to do it more when its windy outside.