Why? Because some people are too damned lazy to say "Lionel". Somehow somewhere people think it's okay or even cool to assume that since they're okay with being "shortened", you won't mind either.
If you're gorgeous and I'm smitten, I won't mind what you call me. As for the rest... Hello, my name is Lionel.
Two plus three? Sigh. My surname is "du Plessis". I do not miss school, college, or colonies of stupid.
I'm not that precious about my name. I'm just irritated with presumption and bad "humour".