Adarq.org

RVJ == SVJ has a very simple explanation, to wit da physiks says

The mass of a body determines the momentum p of the body at given velocity v ; it is a proportionality factor in the formula:

p=mv

The factor m is referred to as inertial mass.

But mass, as related to 'inertia' of a body, can be defined also by the formula:

F=ma

Here, F is force, m is mass, and a is acceleration.

By this formula, the greater its mass, the less a body accelerates under given force. Masses  defined by formula (1) and (2) are equal because formula (2) is a consequence of formula (1) if mass does not depend on time and velocity. Thus, "mass is the quantitative or numerical measure of body’s inertia, that is of its resistance to being accelerated".

http://en.wikipedia.org/wiki/Inertia#Mass_and_inertia

So if your svj = rvj, chances are you're not getting any benefit from the rvj since your inertia is so great it doesn't lead to any significant additional acceleration in the rvj case compared to the svj case.  Or put another way, as inertia goes up (compared to your athletic inertial mass), rvj approaches svj. That'd be my best guess. The closer you are to your athletic mass, the more acceleration you can get out of a running jump.