That was possibly the most interesting part of Halo 2. More interesting in that he was appointed Arbiter AFTER he received the Mark of Shame, which if you watched Halo: Legends, you understand what it was BEFORE that mark became the Mark of Shame. It was originally the Badge of the Arbiter. So, I think that's why he received the rank of Arbiter AFTER he received the Mark of Shame.
That was possibly the most interesting part of Halo 2. More interesting in that he was appointed Arbiter AFTER he received the Mark of Shame, which if you watched Halo: Legends, you understand what it was BEFORE that mark became the Mark of Shame. It was originally the Badge of the Arbiter. So, I think that's why he received the rank of Arbiter AFTER he received the Mark of Shame.