I think the idea is that by trying to actively look for changes, you shift your focus around, but don't actually remember that well what is placed where.

At least for me, my focus kept wondering around the room, never noticing that things that was previously there was replaced by something else.

They were trying to nudge you to rapidly scan over the whole room with quick eye movements, like one of those hidden item puzzles. If they explained what they wanted you to do in too much detail then you probably would have suspected the trick.

It doesn't even say that - it asks you to look for items that are "out of place". When I first watched it I immediately paused the video to have a look. Not the best instructions!

No, I was just kidding.