I think some have had MRI or maybe CT?? where they could place the arms in various positions , and that might show more details of what is compressing things.
Usually if you have the extra C ribs , those are the main culprit, but as others have mentioned post surgery, once the surgeon gets in there they might find anomalies to the norm. Double veins, nerves or veins wrapped around a bone or each other... lots of variations between us.
That's why you need a skilled surgeon that can handle any of those things that might arise.