#1 We do not attend to everything that we see. Visual perception is selective, as it must, for awareness of everything would overcome us. Our attention is often drawn to contrasts to the norm.
#2 Our eyes are drawn to familiar patterns. We see what we know and what we expect.
#3 Memory plays an important part in human cognition, but working memory is extremely limited.
Thinking with our Eyes, from Now You See It by Stephen Few.