I'm not certain, but i wouldn't be surprised if it had something to do with the IMAX presentation of the movie. Even though it was a conversion, it filled the screen rather than having black borders on IMAX Digital screens. Could possibly have had something to do with that.
It's either that or Fox just felt like being cheap and opted for 1:85:1 ratio rather than the wider ratio of the previous films.
Good point about IMAX. It doesn't deserve to be in any cinema screen let alone the grandeur of an IMAX