My opinion is Bungie pull the resolution back to try and slighly increase framerates. 720p with rendering in both low and high range lighting is probably possible but probably results in slightly lower frame rates considering the scale and draw distances in the game. Thus pulling the resolution back probably freed up a little more of the frame buffer to assist with frame rates.
Personally if this is the case I appluad bungie as this is increadable foresight. Having played the game I can say that both the lighting, scale and framerate work well. In fact I think I prefer this lighting system and slightly less frame rate to a full 720p with only high range lighting.
Either way it was and interesting decision by bungie.







