This is not uncommon, but there are a couple of things that can cause it that would be worrisome (if rare). Aortic dissection may partially block the left subclavian artery (artery going to the left arm) and cause the pressure there to be lower than on the right side. Generally you know if you get a dissection (tearing pain in the chest/back radiating downward), but it can be asymptomatic and over time be a serious issue. Its rare but if your doctor is concerned about it they would consider a CT angiogram to evaluate the aorta. Also, since you have had blockages in the arteries of the heart, you could reasonably have a blockage in any other artery of the body, including the arteries in your arms. If you have a blockage (stenosis) in the left subclavian artery this could cause a difference in blood pressure. If you get cramping pain in the left arm with exercise that goes away with rest, it could be due to this type of blockage (arm claudication). A simple ultrasound of the arteries of the arm could be helpful in diagnosing this.
The more likely scenario is that this is just a normal aberration and there is nothing to worry about! Also, if you have no symptoms there's really no reason to screen for the above mentioned possibilities which is probably why your doctor is not very concerned. Like they say...don't treat a number!