I've got a disloyal vassal. He's a Daimyo who controls 2 provinces directly and has 3 vassals who control 1 each. I revoked 1 of his titles....I lost honor. I revoked his other title.....I lost honor again. He promptly revoked a title from 1 of his 3 vassals.....he didn't lose any honor. I revoked that title from him.....I lost honor. He revoked the title from another of his vassals.....he didn't lose honor. This continued until he didn't have any more vassals or titles. So, why did I lose honor every time I revoked a title, but he never lost any when he did it?
Side note to this whole thing, I sure had a lot more courtiers in my court when this was over. Whenever he revoked a title and put that kokujin out on the street, the kokujin and his entire family showed up in my court as new courtiers. When I took the last title away, Mr. Disloyalty himself showed up at my door with his family in tow. I sent him to a monastery (I wanted to slit his throat, but that option never came up on the diplomacy screen).
When you lose your last Kori, you will automatically seize a Kori from one of your vassals, since losing your last Kori puts you in a state of emergency the exchange is free.
You on the other hand, solved a personal conflict through revoking titles, and that is not honorable in the eyes of the game.