Why did Sakurako call Akari a "betrayer"? In the final chapter of Volume 9 of the Yuru Yuri manga, Sakurako
was feeling sleepy in the student council room. 
At that time Himawari stepped in, 
apologizing for being late. But Sakurako, who was in dreaming mind,
saw Himawari's face as Akari's and shouted out 「あかりちゃんの裏切り者！！」.

As I understand it, she is either calling Akari a betrayer,
or saying "Akari's betrayer", but neither of these interpretations
makes any sense to me. What is the real meaning
of these words? Why did Sakurako say that? I know this
manga is not meant to be difficult to understand 
but I just cannot get my head around it.
So please explain that to me if you know the answer. 
I would really appreciate your help!!

p.s. This is the only one place in this chapter where Akari is 
mentioned. So I guess the answer may lie in previous chapters? But
I do not have access to all volumes from 1 up until 9.
The answer for this is actually funny: because Akari and Sakurako are both flat chested, and in anime, girls (particularly flat chested ones) sometimes identify other girls as either friends or enemies depending on their breast size. In this situation, Sakurako thinks Akari is a friend. When she mistakes Himawari for Akari, she thinks that Akari grew big boobs and left her alone being flat chested, and she yells "You Traitor!"

If you don't believe me (it is a bit hard to believe) watch some harem anime. For example, the same situation happens in Negima.