And+since+it+was+certainly+true+that+nothing+of+the+kind+existed+in+writing,+the+animals+were+satisfied+that+they+had+been+mistaken.

Napoleon is manipulating the animals and taking advantage of the animal's stupidity/illiteracy. He always finds a way to justify his actions by fitting the information into the situation, instead of the other way around. By doing this, he always makes sure that the animals, even if puzzled, "accept" the actions and listen to his orders.