Just an oversight. He says at the end of step three that "we have our image divided into 16px tiles colored with 6 shades of grey", but as you point out, if you

Just an oversight. He says at the end of step three that "we have our image divided into 16px tiles colored with 6 shades of grey", but as you point out, if you count the number of shades in the image above, there are more than six.