I honestly don't know how they determine that. A doctor would have to run specific tests. Yes they all run through the thoracic outlet and under the pec minor. My guess is that the reason nerves are more commonly affected is that they aren't as deep as the vascular structures are. Arteries tend to be pretty well protected places in our bodies, along with major veins. This is my guess as to why these two causes are more rare. People with serious venous/arterial TOS typically end up in an ER before they know what TOS even is because of severe swelling/color changes.

This isn't to say that someone with nTOS may not have some construction of the vascular structures and it could get worse over time if patterns aren't corrected...