HELP!

Hey guys, so I am still in the early stages of my process. My I-130 was received on 10/30/2017 and about 2 weeks ago my wife and I received a letter requesting more documents. She was born in Puerto Rico and they werent satisfied with the birth certificate copy we submitted. Below is what they said:

"You submitted a copy of the Abstract Certification of Birth of WIFE'S NAME. However, an Abstract Certification of Birth is insufficient evidence to establish proof that the claimed relationship exists and it's also insufficient to establish U.S. citizenship for immigration purposes. The required documentation is a registered civil authority issued birth certificate that contains the name of the child, date and place of birth, full name of parent(s), doctor/attendant's name, hospital name, registrar's signature and the date the certificate was filed with the registrar's office.

If the birth certificate is not available, submit the oldest evidence to establish that WIFE'S MOM and WIFE'S DAD are the parents of WIFE'S NAME. Submit as much evidence as possible. Evidence may include, but is not limited to"

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I am just confused as to why she would need to proof a relationship to her mom and dad. She was born here. Can someone help? Anyone has seen this before?