"I never get how some people end up with names. We get a lot at work (usually elderly people) where it's "his name is Michael but he's known as Dave" or "her name is Elizabeth but she's known as Carole" I mean they're completely different names, not even a shortened version!"
I think I know how this came about with the older generations. Names used to be less varied than they are now, for example in my family tree there's a preponderance of biblical names (John, Matthew etc.), and 'royal' names (George, William, Elizabeth, Margaret, Mary etc.). Not a lot of originality, but if you've got twelve kids to name you're going to keep it simple, aren't you 
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In my mother's generation, 'Margaret' and 'Elizabeth' were particularly common for girls, those being the two princesses. I had an aunt with one of those names, which was also the name of several of her cousins. For ease, many of them (including her) were not known by their first name but by their middle name. So instead of there being half a dozen Elizabeths, (or a Betty, a Bet, a Liz etc.) there was a Mary, a Jean, an Alice etc.