A: The FreeBMD site works from the GRO records, whereas the local BMD site work from the original register office records. It's worth considering how each set of records originate: Since the start of civil registration in 1837, when a birth, marriage or death occurred, the event has had to be recorded at the local register office. It is from these original records that the local BMD sites work. Every three months, these local records have then been re-transcribed, sent to the GRO where they were copied and re-indexed once again. At each point where data are copied and re-indexed, there is scope for errors and omissions. The level of errors and omissions has been the subject of a book by Michael Foster called
A Comedy of Errors. This book is out of print but may still be available on Amazon
No index can ever claim to be 100% accurate or complete, but by working from the original records, the local BMD sites hope to remove many of the problems highlighted in this book.
The FreeBMD site offers the advantage, however, of covering the whole country in one site, so will allow you potentially, to find ancestors where you may not have considered looking.