Ah was told once (when ah brought it up long ago) that the spaces between the vertical/horizontal is aproximately 1/2 the sensor power (or covers half the square, but because the code only allows all or nothing it shows nothing)
If you want to see in the square diagonal to you, you need an odd number of sensors (ie 3 to see the SSD in the square to the immediate NorthEast or the hungry Krayt to the SouthWest)
Icarus: the numbers are placed kinda backwards, makes it difficult to figure it out properly (ie '8' should be the outer positions with '1' being the centre square, or at least to show the range for the various sensors powers)
{EDIT} Ah do see what you mean now, but ah personally don't understand what the numbers mean in this diagram, always figured if you had sensors to see into that square you saw everything in that square
An ECM 1 ship would be un-scannable where the sensor power is one (the picture depicts sensor power levels), and un-seeable where there are no sensors, etc for better ECM.