These came from a sort of “this sounds like” alphabet. Like if you wanted to write D but drew a picture of a dog, because that starts with a D sound. Or when someone on the phone says “A as in Adam.”
So the word for dude with a tiny hat started with an R sound, just like the word for the A sounded like an A sound.
I have the theory that vampires hating garlic is a rumour spread by vampires themselves because they really love garlic. Getting the humans to season themselves is a genius move.