I too find this interchangeability, even apparent adoption of or being brought up by names completely removed from the given, birth name a real PITA at times.
Lady on the left? Mary Elizabeth. aka " Polly ". Don't ask me how they managed That one!
Uncle Fred was born Thomas. Uncle Bill was Roy. Uncle George still is Frederick, though I can't imagine he'd even turn round if ye called him Fred! Aunty Pip? God knows! I could go on and on.
Why? Why call a kid William, on his birth certificate, then call him Jacob, to his face for the rest of his life? I seriously wonder if half of them even knew their real names.
Oh and Danuslave / Linda; Yeppers. Frank / Francis. I too have even just recently fallen foul of that one! 
LOL your lot sound like mine!
I have the 1911 Cencus for my nanas family - theres only ONE person on it has their correct name, the rest have family nicknames! ............. Luckily for me the only one with the right name is MY nana lol .........
Heaven help anyone looking for James (Jimmy), John (Jack) Edith (Eddie - yeah that ones going to be easy to find!) Margaret (Maggie) Charles (Charlie), Thomas (Tom)............ Neice Margaret (Mags), Caroline (Carrie) his mother in law Margaret (Mary).............
OK the names ARE mainly familiar nicknames, butout of the entire family only Nana got her given name and if you dont put HER name in the search you cant find the others!
On the other side I have a Gt-grandmother who was Born Planceana Mary, but hated her name & used Blanche Mary all her life, her daughter didnt know her real name she died!
HER father was named James Henry, as was his father & Grandfather, and they ALL used James Henry, James or Henry at random throughout their lives, I have to take an educated guess which one id which on records cos theres no pattern to it!
My Nana was christened Doris ............ but always called "Doll" or "Dolly"
My Grandad was John, ................ but known as Jack
One of my Neighbours is Named Geoff ............. but everyone calls him Sam, LOL no idea why
lol and another is named Sydney Mary............. but called Mary at work cos her employer refused to believe a woman could be called Sydney!
Gaille