That is only because the cruise fare for that particular cruise had not changed.
If you cancel a person from a cabin and that makes you the only person in that cabin, Royal will reprice you to the new price, albeit only if it is in there best interest, i.e. price went up.
In your case ((current cabin fare is $3k more than originally paid)) you sliped through the cracks, as a shareholder publish you booking number and I will see that it is corrected 🙂