3ds Max Animation :: Skin Morph In 2013 - Skewing Vertices Side-to-side Locally

The problem I'm having with Skin Morph is that it's skewing my vertices side-to-side locally.

I'm using Skin Morph to better preserve the volume around the two joints of each finger of my hand model (See Arm1).

The two skin morph targets were created for the index finger while it was in the same position as the second image (Arm2).

I repeated this process, creating skin morph targets for the other two joints on the three other fingers and on the thumb.

The problem comes when I rotate the knuckle joint in any axis, in the third image (Arm3) I've rotated the knuckle downwards and you can see the vertices affected by the Skin Morph modifier have skewed to the left. When I turn the modifier off the vertices go back to their original position - so the problem isn't being caused by something like non-uniform scaling of my model (I reset the transforms before skinning).

The fourth image (Arm4) just shows the basic child/parent relationship that I've setup with the Bones and SplineShapes.

The fifth images show the (perhaps strange) position of the Driver Bone Matrix and the Morph Bone Matrix which could be constributing to the problem somehow.

I've uploaded the file so that you can explore my scene. armrigskinmorphproblems.zip

3ds Max 2013 64-bit Student Version
nVidia GeForce GTX 560
Windows 7 64-bit
Intel i7