Why do some people...?

[Andrew Tarr]

And another thing I've seen is "lengthening" names which the original might be considered a nickname.

A Victorian woman (lady) I have followed for a relative was born, and lived most of her life, as Annie, but chose (or someone did) to be married as Angelina.  There's no doubt that it is the same person.

[Sloe Gin]

There's also the option to add a middle name at Confirmation, at least Anglican ones. I was offered (but declined) this in the 60s.

I didn't know this!  I wasn't invited to add a name.  If it had been offered, I would have seized it with both hands.  I hate my name and was only given one.

To be fair, you can call yourself anything you like right now  (either for yourself, or to confuse your future descendants  )

I know. I tried it when I was very young but my family just laughed at me and nobody else would comply. 
I should have done it when I went away to university where nobody knew me.  It's far too late now.

[coombs]

I have an ancestor Mary Newman Smith who wed in 1780 in Foulness, Essex. Yet her children's baptisms say Mary Webb. I guess it was easier to say Mary Webb instead of Mary Newman Webb. She died in 1792 so likely died quite young, and the burial says Mary Webb.

I have never been able to find who her parents were or her baptism, I would say she was born about 1755-1760. When you are getting back that far it often can get harder to trace many ancestors. I wonder if Newman was a family name, or she was illegitimate, or the Newman was after a family friend, famous figure, or local pillar of the community where she was born.

Horatio as a first and middle name increased after October 1805, and Elvis after August 1977.