Nicknames can come from anywhere not just a shortened version of a longer name.
As an eg, my DS is Lachlan and is commonly called Lochie. But I also call him Lockster, Lockabub, Mr Moo, Bubba, Beautiful Boy etc.

It could also be that a nickname used as a baby won't be the same nickname she will have as an adult. She could be Ella as a child and prefer to be known as Anne as an adult.

Have to disagree with you Sarah - I don't think a person should only have one nickname and that doesn't have to be set by one person (ie a parent). I think a different nickname could reflect a special relationship that two people have that isn't shared with others. Anyway, that's my opinion.
I realise this and it is not really my concern, Matilda gets nicknames that do not come from her name and that does not bother me, I am talking about when people shorten her actual name to something that I do not want.

I realise that if I decide to call my DD Annabelle, when she goes to school children might be inclined to call her Belle, she will be old enough to decide if she likes it or she can say 'No, my name is Annabelle/Annie' that will be her choice.

I am mainly focusing on people who ignore what I call my child and decide to rename them! In hospital a particular nurse ALWAYS called Matilda,Tilli, each time I corrected her, her excuse was, oh its my cats name WTF! Just because you like it doesn't mean I do.

Hmm as I said I am a bit picky over these things I think it is alot about respect if people keep doing it when you ask them not too.