Actually, it ‘s because children of Dukes can be referred to as “Princes” or “Princesses”, even when they aren’t the Heir Apparent. It is mainly because most Dukedoms have some kind of bond with the Royal Family, be it by blood or marriage, thus making them royal descendants. Even IF there are no links with the royal family, the children of a Duke are on a completely different level compared to other nobles’ children. It makes sense they are to be called “Prince/Princess” rather than simply “Sir/Miss”. Even Dukes are usually referred to as “His Grace/Excellency” rather than simply “My Lord”. You will find many Novels online which refer to Dukes’ daughters as “Princesses”, for the term is used very often. It’s pretty normal, and not weird at all.
