I think its primarily to do with plane angle. Lets say your plane is at 60 degrees to the ground. For every centimetre you go down, you'll go out half a cm. Say you impact the ball (for arguments sake) with the leading edge at ground level at separation - at separation your impact point should be directly in the (horizontal) middle of the ball on the target line. And say you dig at divot 1 cm deep, then low point (for the horizontal middle of your club/horizontal sweetspot) should be 0.5cm outside the target line/horizontal middle of the ball). Of course it may look further out - by the distance from the middle of your club to the toe - because that's where the outside of you divot is.
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