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.