Why are two identical thermometers giving different readings?

Since I do not smoke in the house, I thought I needed a thermometer for the outside, so I could dress for the temperature. I bought two identical thermomenters at WalMart for $5 each. They were on sale. I set both thermomenters side-by-side outside in the cold. Overnight, one thermomenter gave a reading of 41 degress and the other gave a reading of 44 degrees. Question: which thermometer is correct and why were these identical thermomenters off by three degrees? Both were made in China.