Polar HRM FT7

I have the above mentioned hrm. It seems to have my calorie burn really really low. I work out on an Elliptical for 40 mins. at level 15. On the machine I put in my weight and it also reads my hrm signal so it has my actual heart rate. The machine says I burned 550 cals. the hrm says 272! What?? Because of the wide range (I expect some difference between the two maybe 50-100 cals) I worked out with a friend who has a polar hrm but different model. She weighs 80 lbs. less then me, did the same workout, same level and same amount of time as I did. According to her hrm she burned 420, the machine said 455. My hrm said 270, and machine said 543. Again what?? She weighs less than me yet her monitor said she burned more then me. Oh we both had heart rates of around 138-140 which our ellipticals and hrm agreed on.
When I am done working out I input my info. into my Runtastic app. I put in duration and distance the app (the app has my weight, height and sex stored) the app is usually has the cals. burned a bit higher than the machine but within reasonable range. I usually just input the cals. burned that the machine says since it is the lower of the two. I got the hrm for an even more personal and accurate cal. burn but with it being almost a 300+ cal difference I am beginning to think that hrm's are not that accurate. BTW this is my second heart rate monitor. I had the FT4 before and had the same results. I returned it and got the ft7.
I also need to mention. I have consistantly lost weight over the past 5 months. I eat most of my burned calories which is about 400 cals. over the MFP allowed calories per day. IF the machine was over estimating by 300-400 cals (over my hrm) and I was eating those calories I wouldn't be loosing a pound. I would probably gain. I am frustrated that I spent money on something that is so off. Is anyone else experiencing the same issue??
A side note. I have used the heart rate monitor when I walk. I also use my Runtastic app. I walk about 40 mins. at a 3.5-3.8 mph. The hrm says I burned 238, Runtastic says 245 which is fine they are pretty close. Heart rate during my walks about 135-137.