What causes a 20 point difference in blood pressure from arm to arm? This has been consistent for years. 140/80 to 120/80?

Subclavian stenosis. 10 to 15 point difference is normal but if it is higher than that then start looking for cause such as narrowing of arteries in chest cavity like subclavian , brachiocephalic etc.
Subclavian stenosis. This could mean that you have a blockage in the arteries going out to your arms. This is called subclavian artery stenosis. Many patients without symptoms need only medical management in general for atherosclerotic disease. However, some patients have symptoms of hand pain with effort and even passing out. If you think you could be symptomatic please consult your physician.