I believe that this occurs when there is an annotation or correction (often minor) to the original record or the index.
There used to be staff at FRC and at its predecessor St Catherine's House who went round "interlineating" the books -- adding by hand a note at the bottom of the page referring to an added or corrected entry. Assuming that this has been happening for decades, some of the earlier interlineations will have made it into the indexes as they are re-done (e.g. when some of the original handwritten registers were typeset). With computerisation of records from the mid-1980s, amendments make it more swiftly into the register or index.
