It's very weird: in 35ish years as "Rebecca," I never stopped getting called "Becky" (which I left behind in college) unless I gave another nickname. And yet, when I finally just decided to be "Lucy" at work as well as in fandom and socially, people I had worked with for ages just...switched. No big deal. That said, I do get the odd "Luce." So I do think people just kind of...shorten names and try to find what sounds less formal. Doing when you've asked them not to even once is just rude, though.