Ah, I think I understand now. Its fuuny, because I'm actually due to start learning MEL for work soon.
If that rand() function with those arguments is giving you points randomly distributed within a box, then that is all its going to give you. The two points that you are specifying are actually the corners of the (axis-aligned) box - there is no way to use that function to generate points within a sphere. That is, I think what its computing is: p = rand(<x,y,z>,<x1,y,z1>
returns a point p such that x<=p.x<=x1, y<=p.y<=y1, z<=p.z<=z1. I'm guessing here, but seems to make sense.
So am I correct that what you are looking for is a function that will randomly generate points within a sphere?