"Oh, I see," Duncan said and cleared his throat.
Caius was looking at Alexia. This was the first he was hearing of her being spoken for.
He found it interesting because, whether it was true or not, it seemed the harshest way to turn Duncan down because it felt like, in a public space no less, she was putting the Prince on the spot and challenging him to ask her out still, which would essentially seem like he was trying to coerce her to be unfaithful. By Acheron Empire Etiquette, that was shameful, and even if Duncan got away with it, it would seem like a scandal.
In other words, by saying she was spoken for, Alexia shut down any more attempts the Prince could have made, and that didn't sit well with many in the crowd of guests.
Just as the etiquette frowns on someone publicly coercing another to be unfaithful, it also frowns on not being courteous to someone of Duncan's station. Especially in the role he was in at the moment.
