Why does cheja have a surname after seth but the other two sibling is surnamed after the?

I wonder too, But I think the Cheja is the one who will inherit the Langeres..? Just a thought thouu ╮(￣▽￣)╭

I think I can answer this question...
So in Perseus they need to wait for the oracle, thus who will inherent the position as heir. It could also be if something were to happen to someone the other can assist.

As for Cheja... almost all the langeres with royal blood have silver hair; like the previous emperor and Seth. Perhaps the duke (dragon mage) gave them his advice as who had the most power between the twins in order to decide which to give the last name to.