It could be that the first one is just from a cutscene rendered in much higher resolution and then the second one is from gameplay.
People need to stop talking about the 960x540 video as though that was the source they pixel counted from. Come on, these guys have been doing this stuff for a long time now. They wait until they have a definitive source for counting then they proceed. Those 720p screenshots were their source, not the videos.
If you want to see more of the 720p screenshots, you can see them all here:
http://www.andriasang.com/e/galleries/2010/04/16/alan_wake/