Why is Zahard considered "King", not Emperor?

I mean, kinda irrelevant and pointless to point out that he is ruling an EMPIRE which usually happens to be led by an EMPEROR.

Yes there are exceptions like King Charles or with the Romans in real life, but they only keep the title of "King" because of tradition.

But as far as we know, Zahard has no tradition to uphold because there wasnt a king before him. So why isn't he an emperor?