My left and right arms have the same diastolic bp, but systolic bp in right arm always ~15mmhg higher than left. when under stress (e.g. pulse 135bpm), the difference between r/l systolic bp increases to 27mmhg. why? is it significant?

BP is often higher on the right arm but it's usually less than 10 mmHg. If your left arm radial pulse is as strong as the right, I do not concern. If the left radial pulse is weak, left subclavian artery stenosis can be the culprit but it is rare at your age unless you had procedure done in that area before. A bruit can be heard if the artery is stenosed. R arm BP should be used for assessing HTN.

I agree with Dr. Le's advice, but I recommend consulting a vascular surgeon for evaluation & advice. Such a difference could be a sign of atherosclerosis and I would review your lipid & blood sugar levels. I assume you don't smoke. As has been mentioned, the BP in the right arm( higher) is the valid BP.