Night temperatures can vary quite a bit especially if the wind is calm. The cold air settles lower and closer to the ground. A difference between your two sensors (27 to 31) isn't that huge of a difference. Are they both sited the same? How far apart (vertically and horizontally) are they? IS one in a spot that remains shady? The colder one out in the open while the warmer one near trees/building/fence, etc..?
How far from the airport are you? It doesn't seem too unrealistic for it to vary by that much. I live in Southern California and in the morning on the news temperatures vary from 37 degrees (at Big Bear) to low 70's at the same time only 20 miles away. Lots of micro climates here.