I think you are right. Somebody on another genealogy site asked the GRO to explain. This is what they got back -
"...These are going to be instances where the original registration has been amended in some way, requiring an “occasional copy” to be sent to the Registrar General (ie to supersede the original Quarterly Certified Copy).
They won’t be late registrations, which would be sent in with the Quarterly Certified returns for the relevant quarter..."
The enquirer then asked the GRO if a copy of the original (mistaken) registration would still be available. They replied -
"...If a change has resulted in a re-registration of a birth, the new entry effectively supersedes the earlier one. The earlier entry remains in the index, but would have to be identified in the usual way and a copy of that earlier entry specifically ordered.
GRO would routinely issue the latest entry unless the customer specifically requests and references the earlier entry and if the earlier entry was provided, GRO would enclose a covering letter stating that this Certificate cannot be used for Official Purposes..."
This is as clear as mud. "...the original registration has been amended in some way..." In what way? I understand that the GRO is saying that you could still locate the original registration BUT you would have to know EXACTLY what to ask for. How would you know what to ask for? Baffling.