You might also ask the question, "Why do the corner squares -- still only 1 square away from the center -- rate only a 2, not a 4?"
Thinking of it this way: if you try to access the center square along a diagonal path, you'll only have to pass through a total of 5 scanning points (1, 2, 2), as opposed to crossing directly east-west or north-south, where you'll encounter 10 scanning points (1, 1, 2, 2, 4).
Seems to me that these scanning amounts should be the same, no matter which way you approach a target.
Unless, of course, the graphic is inaccurate, and scanning is more equal than this image suggests.
Also, if the image depicts an entity with a Scanner ability of 8, shouldn't the range of the scan be 8, not 5, as this image suggests?