Hi,
I am not too sure but this is what I think, maybe someone will correct me!!
When a birth, marriage or death was recorded the registrar of that district would record in his books the event, and four times per year he would send copies of these records to the General Registrar of England and Wales. When ordering from a local records centre I have found that my certs are copied onto new sheets in modern handwriting but this may not be the case elsewhere.
When I order from the Family Records Centre in London I think the copies are photocopied from the General Registrars records, which I think were second copies so not the original.
When I order online with scotspeople I can look at the actual orginal entry of a record.