You're looking at two nearly identical grids, certain you're seeing everything... and yet an obvious difference escapes you for long seconds. It's not an attention problem: it's a well-documented phenomenon in cognitive psychology, called change blindness.

The eye doesn't film continuously

We have the impression our vision works like a camera constantly recording everything. In reality, the brain only consciously processes a small portion of the visual field at a time — the rest is reconstructed from an approximate memory of what was just seen. This reconstruction is very efficient day-to-day, but it has a blind spot: if a detail changes while you're not looking directly at it, the brain doesn't necessarily notice, even if the change is objectively large.

The gaze jumps, it doesn't sweep

Contrary to intuition, the eye doesn't scan an image smoothly and continuously. It performs a series of quick small jumps (called saccades), with brief pauses between each jump. A difference located in an area the eye hasn't yet directly "fixated" on can therefore remain invisible, even if it's theoretically within the field of view.

Why some differences are easier than others

Color or shape differences in an area the gaze naturally visits first (often the center of the image, or high-contrast zones) are spotted almost instantly. Conversely, a change at the periphery, or one that doesn't alter the overall structure of the scene (a similar color rather than an object added or removed), requires a much more systematic search.

How to play better with this in mind

Rather than letting your gaze float over the entirety of both grids, mentally split them into zones (for example the four corners and the center) and compare them one by one, methodically. It's counterintuitive — a global glance feels faster — but it's precisely that "wide" approach that lets subtle differences slip by.