I think they do that half-intentionally out of disdain for the rigid European caste system. Yeah. Yeah. I know. At the same time they're falling all over European royalty like a bunch of moviestars. People are not logical. And if it's all Hollywood anyway they don't have to be logical.
Americans can never get titles right because we don’t have them here and we are unfamiliar with how they work.
But it’s like spelling. If you don’t know, look it up.