The simplest answer is a clerical error.

You did not say from where the certificate was obtained - I'll assume it was from Lancashire Registrar.

In this case you are dealing with a third generation document, i.e. it is a copy of a copy.

The original material is with the parish church and an error could have been made there.

Each quarter copies were/are forwarded to the Registrar General. Copying errors are possible.

When one applies for a copy of a certificate a handwritten copy is produced form the Copy in the Registrar's office - again copying errors are possible.

I have received certificates with (to me) obvious errors in dates/ages. On going back to the Registrar an amended certificate has been issued.

The Registrar has never acknoledged a copying error which raises the question -

Was a copying error made or are they just trying to keep me quiet by giving me what I want.