Actually, you didn't understand.

In some situations the light and the background is such that a deer could appear, and if its not moving, will naturally be camouflaged and you may never notice it (ie: see it) unless it moves. This has nothing to do with verifying a deer. It has nothing to do with knowing your target and seeing behind it. It has everything to do with possibilities of specific local conditions in some light.
It has to do with knowing the deer is there in the first place. Once you know he's there, he's as apparent as can be. But in low, flat light, with the right colored background the deer completely blends in.

For instance, the blind I was sitting in tonight. At dusk, if your looking off to the right, a deer could enter the field from the left 50 yards from you and wind up standing in front of some hay piles. The hay is the same color as the deer. In flat light, if the deer doesn't move, it will be perfectly camouflaged and you may never know its there when you look back to your left . You would have no problem seeing the rest of the field, as well as the fact that there is a hill behind the hay. And of course, since your shooting on a downward slope a double lung shot will hit the ground about 6 yards behind where the deer stood.