I have a minecraft arm mesh set up on the right, with the shape keys working at a value of 1. On the right, I duplicated the mesh first, then the rig over from Edit Mode of the rig, then connected them. I still had the old shape key on the mesh because I don't want to go through all the work of making the mesh perfect again. The second mesh on left, with a shape key ArmB.R at a value of 1, distorts the mesh. They are both equal at 90 degrees. With my knowledge, they both should look like the mesh on the right when both shape key values are at one. Where did I go wrong?
For a closer inspection, I have uploaded the .blend file.