^ Because she's the one who insisted on cutting the baumkuchen into eight pieces, and she also had access to arrange the baumkuchen pieces in such a way to make one piece look bigger than the other (Jastrow illusion).
On Serika's death, I've just read about ゲシュタルト崩壊 ("collapse of gestaltism") in 2chan - gestaltism is where you see an object as a whole rather than a collection of its constituent pieces, but when you look at that object long enough, gestaltism "collapses" and you start seeing the object as a collection of pieces, and you end up wondering about how what that object looks like. Unfortunately, there's no English version of the Wikipedia article for "collapse of gestaltism", so I don't know what the English term is, but apparently it's well known in Japan. See this Japanese YouTube video collapsing your gestaltism - this video essentially explains that, when you look at one character for a long time, you start wondering about how to write that character.
That explains a lot! The secretary also made the wife look at all those logos for a long time, so much so that the wife started doubting how to write her own name. Interesting . . .