AOTC was not shot at a higher resolution than 1080p. I'm not even sure it went that high. It was digitally upconverted to be big enough for IMAX. Must have looked like shit.

ROTS was shot with the next generation of the same camera, and not even that camera could do higher resolutions than 1080. In fact, ROTS was quite simply cropped from its native 1080 format (i.e. it was shot with non-anamorphic lenses), so that it just used 817 vertical pixels, according to the CineAlta wikipedia page.