HRM and Heart Rate while Running

So I'm using my Polar F6 HRM b/c I wanted a more accurate calorie burn count during exercise, particularly running. I'm 33, 5'4, 138ish....I run 4.5 miles in 48 minutes and I burn 340 calories? Shouldn't that be higher?? and that was me pushing it a tad more than I usually do but I can't get my heart rate up real high -- 137 was my average. My heart rate recovers well and I'm just assuming that my heart is efficient?

Thoughts? I know we are all different but my running BFF uses the same HRM, weighs the same as me, and her heart rate is waaay higher thus showing a higher calorie burn. I'm stumped. I just feel like I would be burning more....am I going to have to pu**** that much harder to get the bang for my buck during my runs?

Are you on any kind of medications? I'm on a beta blocker and I can't get my heart rate up past low 140s even if im pushing as hard as I can. I'm not sure if that translates to fewer calories burned or not :-/

it might be a good exercise to get your zones/anaerobic threshold/max HR tested at a gym or testing facility.  without that test you may not know what you are capable of HR wise etc.  as far as the HRM calculation of calories, when i use my Garmin HRM, the calculated burned calories are significantly lower than any online calculator (like those used in most popular apps etc).