Re-Registration is a complex subject, little understood by most researchers (myself included, before I worked as a registrar).
Births can be re-registered for a number of reasons - and often are - the most common being to add an unmarried father who was not named on the original entry, and to legitimise a birth after the later marriage of the parents.
The main thing is that a re-registration creates a new register entry, which can be months or often years after the original event (an 1892 birth being re-registered in 1939 is the longest I have found so far). The new entry can be in the same name, or a different name(s), depending on the circumstances.
Corrections to entries, which are a completely different thing, do not create a new register entry, although they might mean an amendment to the original index entry.
It is possible, and advisable, when researching such a birth to get copies of all the relevant entries.
