What you are looking at on-line are just indexes to birth registrations. Those registration documents are paper records that only exist in the vaults of the relevant local registration office.
Any certificate is just a copy of the information contained on those register entries.
How the indexes work varies over time, but before 1969 birth registrations (in England/Wales) don't show any surname for the child, so are indexed by one or both of the parents surnames depending on their marital status.
Without the details, and when this occurred, it is impossible to say what you are looking at, but it may be a birth that has been re-registered, so it appears more than once in the indexes. It is possible (and quite common) for a birth to unmarried parents to be registered first without the father's details and then again later to add the father or after the parents later married. Each creates a new registration and a different entry in the index.