For AD629, is not functioning as unity gain amplifier. Pl. help

For AD629, the output of the differential amplifier is not the exact difference of both the input. It is slightly higher.

For example: pin 3 = 2V, pin 2= 1V, pin 6 has to be 2V-1V=1V but observed is slightly greater than 1V (i.e. 1.2V or so - exact value not available). Thus it is not functioning as unity gain amplifier. Pl. help us to understand as to why we are seeing such a variation in output.