Why is June 유월, and not 육월? And why is October 시월, and not 십월? It would make more sense for them to be 육월 and 십월 due to 6 being 육 and 10 being 십, right? Well, not really.
There's a reason why these words are the way they are, and it has to do with them being easier to say that way. Find out more in detail~