Question about calories burn w HRM

dkhuff1

I just started using a new polar FT7. I had a polar after six that is about 10 years old. I noticed when I do the exact same work out with the same intensity, the calorie burn on the new FT7 is half what it was with my polar F6. My calorie burn with my F6 used to track very closely to what my fitness pal would calculate based on the exercise. Now my heart rate monitor registers half the calories. I did check and make sure that my weight is correctly put into the new heart rate monitor and it was correct in my old one. Any ideas why these would be so different?