I am sorry if I am posting in the wrong subreddit. perhaps I should post it in ELI5 instead? I already tried searching the web for the answer, but no luck.

I have notice that sometimes there is no "of" in the titles of nobility, like people below:

http://en.wikipedia.org/wiki/Robert_Stewart,_Viscount_Castlereagh Shouldn't it be Robert Stewart, Viscount OF Castlereagh? since Castlereagh is a place?

or in http://en.wikipedia.org/wiki/Walburga_Habsburg_Douglas. Should it not be Walburga Habsburg, Countess OF Douglas? Is it because Douglas is a last name?

Thank you so much!