User blog:Lvdoomien/The Names of OP

Lvdoomien here, and I'm discussing the weird names of the characters in OP. And no not the strangeness of their names, like Ganzack and Bellemere, I mean why so many characters have their surnames (last names) in front of their normal names. For example Roronoa Zoro and Monkey D. Luffy have their last names first and I'm sure some people have thought thats Luffy's real name was Monkey and the same with Zoro like me before I figured it out. But what I've been looking for is insight on the simplest question: why? Why the surnames first, and also why are some people's surnames last, like that Dadans? Anybody have any idea?