My guess is they want the primary button to always be on the right side. For a first time visitor that primary action is always to create an account. If the user is a return visitor, they assume the user already has an account and so they want the primary action to be to login to that account.

I take it some UX-er got paid handsomely, but I think all it does is confuse people. As someone suggested already, I’d just go for login/register and make login primary (since you only register once, and login maybe hundreds of times after that)