First off, I don't agree with the statement about the VG, but be that as it may, I think I can explain it to you.
A ball only has the energy applied to it by the bowler. The coverstock, surface finish, interior construction and layout all function to determine where and when that energy is used, if at all.
A ball for heavy oil generally rolls early, using some of the energy to get the ball started in its hook phase earlier, leaving less for the backend. It may cover more total boards, but less on the back part.
A ball for lighter oil generally skids further, saving more energy until the back of the lane, resulting in a large backend movement.
A heavy oil ball used on a light oil condition will use all of its energy early and never hit hard.
A ball for light oil used on a heavy oil condition will never use its energy on the lane and just spin into the pit.
The trick is to get the combination right on a given lane condition.
Hope this simplistic explanation helps you.