Hi Tae,
Did you try to measure the supply voltage and check if it's really 3.3V? Since the output offset voltage is dependent on the supply voltage, a slight difference in the supply would change the output offset voltage. Also, how did you measure the output offset voltage? It is stated in the simulation mode section of the AD8555 datasheet on page 22 that the first gain stage has a maximum tolerance of 2.5% and the output offset has a maximum tolerance of 0.8%.
Regards,
Jay