Maybe someone else will have more specific ideas or can point to a place where YNAB has discussed the reasoning... but I'm assuming it's to fit within YNAB's 34 day free trial period so anyone can join even if they aren't a subscribed YNAB user right now.
Slate Gray Cobra that's an easy one! Its so that no matter when you start your free trial, you are guaranteed to experience how YNAB handles rolling over from one month to the next. (That said - why 34 days exactly instead of 33 or 35, I don't know. 😄)