I just found the reason for that. In the .cpp (source) file of that INA3221 library, 5uV has been used as the sensitivity , where the real value is 40uV. Also in line 418, the variable "val_raw" should be right shifted 3bits as follows. And it should be casted to (int32_t).
res = (int32_t) (val_raw >> 3) * 40;
You could search the .cpp file of the library, typically available in Documents/Arduino/libraries folder. Then edit the INA3221.cpp file.