Many reasons for omissions. Bad writing, mistranscription, loss of records.
In the case of my Cullimore item, due to bad writing it was Collimore at the GRO, but being a local name, the Welsh registry recorded it as Cullimore.
Not quite the period of your problem but if you read 'Comedy of Errors' by Mike Foster, a New Zealand academic who was allowed to examine the records and who wrote a detailed and scholarly analysis, you will find he estimated 1m names are omitted, mistranscibed etc from the GRO indexes between 1837-1900.
Surprisingly he was allowed back for a second visit resulting in yet more alarming revelations, and if memory serves it was then that he found index slips between index drawers and the cabinets ... .
He wrote a second book about that visit.
Digitisation of these records is long overdue.