Renewing child passport - why has office requested birth certificate??

[Milomonster]

My ex DH applied to renew DS’s passport as it expired. He is 10. All requested documents sent and the passport office have asked for DS’s birth certificate. Why not request it in the official list of documents to send?
Ex and I have different surnames if that’s useful info.

Did you apply for it last time?
Might be because it's your ex dh and they are checking he has parental responsibility for your ds