Win7 64, gtx 580
Blender 2.73 official
The preserve volume feature of the armature modifier gives incorrect results when scaling a bone.
See the blend file: a cylinder is rigged with 2 bones. When rotating the second bone, the deformation is not the same, depending on the scale of the first bone. I've duplicated the armature and the mesh on the right, and scaled the first bone, so that you can state it clearly.