There were actually multiple aspect ratios of the film distributed. What ever the native size of the theater's screen was the version they got. The idea was to have the largest picture area and no masking.
Very few (if any) theaters have a native screen size of 2.35:1. That's very limiting. Besides that, the "film" wasn't shot in scope format. The few scenes that aren't 100% CGI were shot digital, which is natively 1.78:1.
Cameron was VERY fussy about the image size. In fact the IMAX version of the film didn't fill the screen horizontally. (or obviously vertically either).
