Some 'extra' names are those given at baptism. My Dad said it was common practice in Catholic families to add a Saint's name. I guess some could also be in honour of the godparent(s). I have known one case where a chap started using his father's name, which would have been shortly after his father died, so i can speculate only this was in honour of his father. I have also come across an immigrant family (in a tree I was compiling for someone else) where none of the births were registered with a forename. This threw me for a while - usually you see 'male' / 'female' where the baby died. Luckily for me, this family was in London, and Ancestry 'hinted' at the baptisms and there were enough children of this marriage (!) that even I spotted the correlation between baptism record and anonymous birth registration.
As to death certificates, a general point is that the information on them is only as good as what the informant provides, not necessarily therefore what the deceased would have said.
I'll come back to the 1911 Census - time marches on!