It has to do with 'Dwell-time' of the bullet in the barrel. Hard to quantify as the differences are in milliseconds. Just as a heavier bullet with the same charge will print higher due to longer time in the barrel(Dwell-time) because of the lower velocity and more inertia two bullets of the same weight loaded to different velocities will print at different vertical points of impact.

Using a Garandicon as an example, .008 changes point of impact 1-inch at 100 yards(Actually 1.08"). A few milliseconds longer in the barrel can cause a raise of elevation of as much as four or five inches due to the longer stretch of recoil. You're hardly moving the barrel upwards at all to achieve this. Less than 1/2 of an inch.
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