The original Birthright materialcasts Heirl Diem as the current Baron of Diemed. Now, considering that his line follows a direct line back to Diem, first Duke of Diemed, and that even his recent ancestors were dukes, why is the Baron a baron?

Whilst pottering around the wiki, I also noticed that the Baron is described as Duke of Diemed in several places, notably on his own page.

So, what is going on? Why is Heirl Diem a baron, why wasn't this duchy/barony confusion explained anywhere and why do some people clearly feel that he is a duke, in contravention of the source material?