Many in the Empire have a curious stance on the notion of lying; in many courts, it is not considered dishonorable to lie so long as the individual speaking genuinely appears to believe what he says is the truth. This is a skill that many have honed in order to defend themselves from the predation of more skilled opponents in court. Even bushi attending court as yojimbo often practice such things, to ensure that they are not viable targets of their charges' enemies. This can also be used to avoid offering insult.
Honest is used when you must speak the truth in a convincing manner, while Deceit is used when you must tell a lie and appear to tell the truth ("I assure you, magistrate, I was meditating in the shrine well after sunset last night"). Sincerity can also be employed to tell the truth in a very cautious manner to avoid giving insult (when a morbidly obese governor asks a samurai if he is not the greatest warrior in the Empire, the samurai cautiously respons with "All other warriors would surely vanish within your might shadow"). Rolls made with Sincerity can be opposed with Investigation by those who doubt the truth of the speaker's words.
Note: Mastery abilites ranks 3 and 7 are house rules and are not part of the standard 4th edition ruleset.