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AD7414 - Seemingly Inaccurate Temperature Readings

The AD7414 on my board appears to be reading temperatures 10-20 C above the actual temperature.  The attached plot shows the AD7414 output in blue, and three thermocouples placed near the chip as the other three lines.  The test was repeated with two separate sets of thermocouple devices, placing thermocouples directly on the AD7414 case, the AD7414 ground pin, and the board surface next to the AD7414, all showing the same results. 

The chip is operated at 4.1V, so I expect a maximum +/- 3C error, as described in the datasheet.  Any idea what may cause the difference in measured temperature?