I have no idea.

I can find several Cavendish mothers but no fathers or children.

I know there are about 20 Braddan births missing but they are consecutive entries in the book, so that is not the cause.

I see two possible explanations:
1. The family did not bother to register, and I suspect this may be the answer as I also have not been able to find several persons who show up in the 1881/1901 censuses, generally with more than one child in a family.

2. There are Registration books that were not filmed.