I think the main question was, why is the RG increased in a lighter version of the same make of ball. A higher Rg ball requires more effort to rev than one with a lower RG.

First, RG is defined as the distance from a ball's axis of rotation to a point where the total mass of the ball is actually concentrated. For a sphere of uniform density, the total mass is concentrated right at the geometric center of the sphere. For a bowling ball, its total mass can be shifted away ( and more towards its shell) in several ways.

One way is to just decrease the the mass of the core. This will shift the center of mass towards the shell, increasing RG. In reality, there are a number of ways to increase RG. Decreasing the ball's core mass ( but not necessarily its density) is one way. RG increases, the farther away the ball's mass is shifted from its geometric center. For those of you who are interested in the "math", the GENERAL formula for calculating RG is :

K (RG) = ( the square root of 2/5) X (the radius)

If a 14# ball had the same RG as its 15 # version, its performance would not be quite the same. In order to TRY and replicate the amount of torque applied by the hand in a 15 # ball v a 14# ball, it becomes necessary to raise the RG value in the 14-pounder.