AD8228 offset issue.

Dear Sir:

                We used AD8228 for shunt current sense application. We have some confuse from AD8228 output offset. As below picture we tried to measure same current on different common mode voltage and it had different current offset , Would you kindly help us to find reason for this result? Below is our detailed conditions :

X axis:

1=100uA

2=200uA

3=500uA

4=1mA

5=10mA

6=20mA

7=100mA

8=200mA

9=500mA

Y axis: output offset

AD8228 conditions:

VS=7.5V

-VS=-5V

REF=2.5V

Best Regard

Jason