The accuracy of the device and the external probe are tested under stable laboratory environment. When used in real cases, the readings will be affected by the conditions.
We conclude some possibilities that could increase or decrease the temperature readings:
1) The sunlight. If the device or the probe is exposed to direct sunlight, it will be heat up.
2) Contact material. Is your probe hanged over the air or placed on the ground? The material it contacts will also affect the temperature transmission. For example, in the same room, the temperature of the wooden desk surface might be warmer than the iron desk leg.
3) Ventilation. This is also an important factor for the temperature. With better ventilation, the temperature is lower. Since the device sensor is very sensitive, they can detect tiny air changes.
In conclusion, we recommend you place both the device and the probe on the same place and wait for half an hour. Then they will come out close readings within the tolerant accurate range.