For the Tpose it’s the same reason as described above.
as for the “why isn’t blender” part: mostly because the values in blender aren’t truncated.
assuming they were only when below .001 you would probably see the same result.
Also, As I was re-thinking about this:
If you eliminate the 2 bones on the shoulder and make a proper anatomical rig for your meshes you would very likely eliminate the distorsion - Given the distortion is generated by truncating the value on only one bone you have a 50% of eliminating the right bone and not seeing the distortion even with the stock engine.
You know you can generate the correct right with Rigify in blender right? You actually don’t have to overwork the armature to get it just perfect…