New HRM, confused about the calorie burn?

FionaAnne22

Hey all, so I got a Polar FT7 a few weeks back and love it, been using it at the gym, swimming etc and it seems great, however there is one thing that puzzles me with it...I am 215 pounds, and was at the gym today for two hours, doing cardio and kettlebells. Previously, I would have logged this as about 800 calories I guess, but according to my HRM its only 600 odd. I trust the HRM, just seems that I'm burning nowhere near as much as I expected!

I'm not sure if this means I'm fitter than I thought? Just a bit confused about it! Anyone have the same? or ideas as to why its low?

Thanks