I had researched the late 19th and very early 20th century birth ledgers of my home county for several years before I realized what they were: transcriptions of the original birth certificates submitted by the attending physical, midwife, or occasionally parents. What was contained in those early ledgers was a handwritten copy of the certificate.
I also discovered that in the early days of recording births in my county, the certificates were mailed or taken to the courthouse where they were retained and the information written in the ledger. This meant that technically during this time period, the ledger was a derivative source–because it was derived from the original certificate. That does not mean there were incorrect transcriptions in the ledger, but just the potential.
In some cases, the ledger was more legible than the actual certificate. In some cases, it was the other way around. In some cases, there was an additional comment on the certificate that was not written in the ledger.
It always pays to know what you are searching. In this case, the certificate is the original document and the ledger is the derivative document.