why mom need to go to Shanghai for interview (application throug

mom has come to the U.S. many times. Every time she goes back to China on time without extension.

Mom just submitted her B-2 application through Nan Jing CITIC (Shang hai district). Yesterday, mom got a letter, saying "bring her old expired passport and/ B-2 extension and go to Shanghai for an interview".

Previously, mom and dad applied for B-2 together and got their visa. This time, only mom applies. This may be the only difference. Does it matter?
I do not know why? Any comment is appreciated.

Thanks