Another thing to keep in mind is that HR doesn't actually indicate calorie burn. Calorie burn is a heat measurement (at it's most basic) and HR doesn't necessarily have anything to do with heat. If you get scared or nervous (or sick), you could have an elevated HR and yet your actual calorie burn isn't any greater.

HRM for training is great, but it's only a tool and only as good as the algorithms within it! I've heard they are more accurate for men than for women, but again, it's an educated guess at best. In my case, I have an abnormally low HR and I do find that my HRM is pretty good at determining calorie burn in most cases. I have noticed that the longer I ride, the worse the accuracy is though. If you have a more elevated HR, then the calorie burn is likely going to be inflated.