After first viewing, I assumed he mixed it up in post to help create more tension that he felt he hadn't captured. Later I discovered that it was written that way. Though I used to feel that it was arbitrary, I do honestly think that it changes the way you experience the film in positive ways. You really learn about each character in a very complicated way, and the reveals built into the structure are well-timed to add suspense and even helpful confusion.
That said, I agree with Ebert (never again shall I say that) in that Innaritu doesn't need to do all of this, and that he'd be a great filmmaker just letting us fall in love with his characters and suffer with them. He captures a rawness of life few can deliver. Ilke Ozu, Ebert suggests, he may mature by removing elements of style from his filmmaking.
By the way, Rodrigo Prieto has too much talent for anyone's good, I hate him for it, and I'm pissed that the academy is screwing him this year.
Will
You may like grandma's yard gnomes, but I've seen Rock City. Remember it.