Your visual system (well really, all of your sensory inputs) have an effect called adaptation, where they "get bored" of any constant input. Your ears stop telling you about a constant hum, your skin stops telling you about the weight of your clothes, etc. So all the parts of the images that are the same, your optic system gets tired and just stops "seeing" them. So you can literally only see the parts that are different.

It's a partial version of this effect: How to Blind Yourself Temporarily https://news.ycombinator.com/item?id=3992208