RFE Letter for birth cerficiate

I am applying green cards for my parents and received a RFE letter requesting additional birth certificate to demontrate the relationship of my parents and me. It stated that the notarized birth certificate is a secondary evidence. I read the notes in similar situation and the advise is to resend the notarized letter again.  I am wondering why they would accept it. Any help you can offer will be greatly apreciated.