I'm not a driller or anything, but from my observations, I believe the following:

http://putfile.com/pic.php?pic=7/20923341499.jpg&s=x1

You can see this on bowling balls where the shape of the core is shown to mark the mass bias.  Putting it inside the VAL tilts the core in such a way that is less stable and therefore will take a longer time to spin on its preferential spin axis.  

Putting the mass bias beyond the VAL will tilt the core such a way that it's almost already rolling on its preferential spin axis.  It will take much less time to find its preferential spin axis, and because it finds it earlier, the ball will go into a forward roll much earlier.
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