The way he approached the EoY Special exam recently was similar, no matter which class won or lost he was planning to intervene accordingly. But he failed to anticipate that Ryuen and Arisu would bet that the loser would leave the school.
Except that he did anticipate the bet. This has been hinted at all the way since Y2V6 and as well as Ayanokouji’s knowledge of it. This is newer analysis but I’ll share some of it with the explanation of why Ayanokouji did know about the bet:
In y2v6, Arisu and Koji had their conversation.
Here are some statements from the volume, in contrast to statements made in y2v11.
Arisu: “So you’re trying to create the strongest class with your own two hands, Ayanokouji -kun, is that right?”
Koji confirmed this, and Arisu replied:
I can’t say that I haven’t considered that before, but… it does leave me with some questions.”
We will skip a bit, to the following statement by Koiji:
“I’m living my life at this school according to my own plans. The fact that you came to my room today, and that you showed interest in my plan, along with the possibility that you’d arrive at the answer, and that there would be a route to arrive at that conclusion - that was all part of that plan too.”
Basically, Koji prompted Arisu to think about and rationalize his plans in this way. What way exactly? It was revealed to us in y2v11:
“The competition between the four classes that Ayanokoji idealized had become impossible when the bet with Ryuen was decided. Even if Ayanokoji transferred and steered towards balancing the forces, considering Ichinose’s declining class, there wouldn’t be enough players”
Quite the jump to make. From thinking Koji is looking to build the strongest class, to a 4 way battle.
In other words. Koji knew Arisu will eventually figure out his plan and try to sabotage it. He provided her a way to sabotage his plans while simultaneously finding a solution by creating a way to blackmail her later (special lesson)
So, how does this all fit in so far?
We know the Arisu Ryuen bet was changed for a few reasons:
It doesn’t fit Katsuragi’s personality, there’s no way he’d agree to such a deal anyway, and it was stated he is unaware. (And honestly seems kind of odd. Ryuen has potential to get 3rd place, and he randomly agrees to an offer to bet his life in the school? It’s risky, too risky, even for Ryuen).
Arisu didn’t fully understand Koji’s plan during the Island exam. It was shown to us in y2v6 - she confirmed that she believed Koji is trying to build a strong class, not balance the forces between the classes. How does taking down Ryuen help sabotaging that?
So, why did they wait so long?
After the events of y2v6, Arisu figured out Koji’s plans and proposed Ryuen to change the contents of their bet. But at first, Ryuen wasn’t a fan of the idea. Like I stated earlier, I believe this bet is too risky even for Ryuen. So, why did he change his mind? The answer is Hashimoto.
In y2v11, Hashimoto stated the following:
“Of course, I didn’t take such an invitation seriously at first. However, right after that, I heard from Ryuen that if I didn’t move classes, I’d definitely regret it at the end of the school year.”
Ryuen waited for Hashimoto to make a decision. And Hashimoto indeed chose in y2v10, which prompted Ryuen to accept the new bet after the events of the exchange camp.
I’d like to make one more point, before I end this analysis.
In y2v9.5, Hashimoto approached Koji with an offer to move up to class A.
Koji: “If she’s going to pull me (he’s talking about Arisu here) to class A, it’s more than a great proposal.”
Hashimoto: “That’s what I mean. If class A is inviting you, would you accept? Even if it’s just hypothetical, let’s hear it.
Koji: “I’d positively consider it if I was offered an invitation to class A.”
We can see here that Koji pushed Hashimoto to beef with Arisu over his transfer. Koji had full analysis on the whole situation. He knew how Ryuen would feel about the deal, and that he’d approach Hashimoto since they both had analysis on Hashimoto’s character. And then, Koji said what he needed to say to push Hashimoto in that direction. He orchestrated the whole thing, and ensured it all went according to plan.
So at first, Arisu proposed a different bet to Ryuen, which was accepted by both him and Katsuragi. Koji knew Arisu will eventually come at him, and try to sabotage his plans. After the Unanimous exam, he realized Arisu and Ryuen plan to battle it out, and made some sort of a bet. He then planned a conversation with Arisu, where he induced her to reason his plans in this direction. He made her connect the dots during their conversation, as he himself stated later.
This got Arisu to suggest a bet change to Ryuen. He was not a fan of the idea at first, but After Hashimoto chose his side, Ryuen felt comfortable enough to accept the bet. Due to the change in bet, Katsuragi isn’t aware, and the reason it took them so long to finalize the details is because Arisu changed the bet after y2v6.
Koji orchestrated the whole thing, which means he already had a way to nullify the bet since y2v6 - infact, he created it right then and there. Both Arisu and Ryuen are key components in his plan, he won’t let them get expelled.
Then as of Y2V12 this was further confirmed when Ayanokouji mentioned how he had prepared different strategies for a variety of outcomes regarding how the EoY special exam could play out.
He foresaw the fact that the class leaders could engage in a competition, and therefore he prepared his Sakayanagi contingency against a competition which he envisioned after the whole thing in Y1V10 when Arisu lost by doing his conversation there which influenced her mentality in the future, knowing from the start she'd win the final exam
He said he didn't predict them to actually go through with it to the point of expulsion NO MATTER WHAT HAPPENS, which means his passive strategy which would have succeeded without their bet is already destroyed, therefore he has to use his contingency
This also shows that Kiyotaka has some sort of abduction regarding how final exams work every year, seeing the one from Y1 + what Nagumo told him. This is supported by him saying: "no matter which class wins or loses, I have strategies prepared in advance"
Sorry for the long response on this but I wanted to show the connected events back to Y2V6 which show how the bet DID NOT catch Koji off guard and to the contrary was something he anticipated and prepared for long in advance.