Why do the king and duke add $415 of their own money to that hidden in the cellar?

The King and Duke discover they have received the bulk of the estate holdings as well as three thousand dollars cash. The three girls have also received three thousand dollars and the house they live in. Wilks's will tells them where in the cellar to find the cash, and the two men go downstairs and find it. The King and Duke count the money and come up four hundred and fifteen dollars short. To alleviate any suspicion, they add the money they made from the Royal Nonesuch to the pile. Then, to permanently win the town over to their side, they graciously give their share of the money to the three girls, knowing they can steal it back at anytime.