From what I've heard, the iPhone does some pretty cool accelerometer processing behind the scenes to find the frame which is the least blurry. From this post, i

From what I've heard, the iPhone does some pretty cool accelerometer processing behind the scenes to find the frame which is the least blurry. From this post, it sounds like it might be storing images for processing before you trigger the shutter, creating a buffer around when you wanted the shot. This makes a lot of sense, especially since the act of pushing the soft shutter button moves the device and could cause a blurry photo.