Even if a doctor is the only one who actually signed a birth certificate, there were other informants. The doctor (or midwife) did not provide all the information from their first hand, direct knowledge. The doctor or midwife would have probably known the details of the birth (date, time, place, mother, etc.) The parents likely provided their names and any other information about themselves listed on the certificate. The difficulty is that in records with probable multiple informants, it’s impossible to know exactly who provided which pieces of information. That doesn’t mean the information is correct or incorrect–it’s just that we need to think about who most likely provided it.
And some of those pieces of information we won’t be able to know 100% who provided it–we weren’t there.
And if you are insisting you were there when your great-grandmother’s birth was recorded, it may be time to focus on things other than genealogy for a while.