Why is blood pressure variation of 20 mmhg in right nd left arm?

Bp: You can have up to a ten point variation any greater may represent an arterial narrowing in the arm arteries ( the arm with the lower pressure ) found often in smokers.

Subclavian stenosis: Typically caused by subclavian artery stenosis. Left more common than right. No treatment required if asymptomatic unless coronary bypass with lima is planned.