I've always taken this to mean the amount of volume in the front part of the lane.  So my interpretation is that a fresher "hard block" would be an oily wet-dry, while a more broken-down house wall would be a hooking wet-dry.  On the former condition, you'd encounter over-under more in the front part of the lane, while the latter condition would be more over-under near the breakpoint.
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"Dispensing conventional wisdom to a fingertip world."

Horrid in Doubles, torrid in Singles...
...that's The Curse of Dusty.