AD620ANZ Instrumentation Amplifier acts as an single operational amplifier.

Hi, I don't know, if here is the right place to ask this kind of question but i have a weird problem about AD620 IA. I purchased 6 (six) AD620 and tested them if they are working. All of them did not seem working as an IA. But wen i decided to try it to work as an single opamp, they worked. I tried them gain of 10 and 1000 with inverting and non-inverting mode. Is there anyone met this kind of problem. They are working as an single opamp not an instrumentation amplifier.

The package marking on devices are all same, the marking is: AD620ANZ #1839 1780933