Typically, on a animated television series the enshutsu will provide the storyboard for his episode because the episode is his own work. The storyboard credit is often assigned to another if the enshutsu is not a good artist, but even then the direction is coming from the enshutsu. While the 監督 (Kantoku) has final say episodes are usually a reflection of the personal style of the enshutsu.
Secondly, the 作画監督 (Sakuga Kantoku) credit is better translated as Animation Supervisor, not Animation Director. While kantoku does mean 'director' the role is not actually a directing role. Going back to my previous example: Iwane Masa'aki does not direct so much as he supervises and corrects. While an Animation Supervisor does help with adjusting timing and the like typically what Iwane will do is check the key animation drawings for being off-model or perhaps bad timing. Of course, Iwane typically provides all of the key animation for an episode himself so that usually is not a problem. Iwane will also work with the episode director, Asada Yuuji, as needed. I suspect Asada and Iwane are quite close and Iwane has quite a bit of influence beyond simply drawing all of the key animation. I should note that the key animator also decides the timing of the in-betweens, but that is not really related to the credits.
I can understand how this might have been confusing based purely on the kanji of the credits and not a deeper understanding of the roles these credits actually stand for, but that's why I like to help. I think it is important that this is rectified for the sake of spreading cool knowledge.
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