My mum used to do this with me. She hated the common shortening of my name, and it was completely forbidden, throughout my childhood. To be honest, I didn’t even dream of questioning this, or of thinking I might have the right to choose the name I wanted to be known by.
When I started nurse training, there was another girl in my set with the same first name as me, except that she was normally known by the common shortened name, so she kept that, and I kept the full name, so people could tell which one of us was being referred to, but when I left nursing after 6 years, and went to university, I decided that I wanted a fresh start, and a ‘new’ name was part of that, so I’ve been called the shortened version ever since.
I eventually got up the nerve to ask mum to call me the shortened version, and she was not happy - though she said she would ‘try’. She managed to use it sometimes, but it never felt as if it was natural for her. She passed away last spring, and I am pretty sure she thought of me by my full name right up to the end!
Looking back, it does seem as if she thought my name was her decision, and I had no ownership of it, but I cannot explain why.