This feels incredibly counterproductive for learning these terms, particularly given the variation in the phrasing. What is the point of presenting them out of order and interspersed with other random terms? Between this and the made-up radical names, I’m starting to question the philosophy behind the way things are presented here, and it does not make me want to subscribe.
I think the order is based on the complexity of the kanji and the frequency of use. Also WaniKani isn’t meant to be a stand alone Japanese language learning tool. People usually combine it with a textbook or online lessons where the months would be taught in order. WaniKani just helps reinforce the kanji.
Maybe the better question is: why would it be any more productive to learn them in order?
I would get it for another language where there is no pattern, as learning them in order can help create an internal sing-song for it.
But for Japanese, does it really help to learn 五月 before 六月? They’re numbers based and don’t really use any non-standing readings. (There are some other names for them, but aside from 正月, they’re not really used and even that one is really just used for the first few days even though it can be applied for the entire month too)