Your HRM is deceiving you.
The true measure of calorie burn is how much oxygen you are consuming. Unfortunately, this is hard to measure outside of a lab. While heart rate normally increases in line with oxygen consumption (and HRM's use this assumption to estimate calories), there are factors that can cause this relationship to break down - high blood pressure and some medications are just of the more common causes.
The limiting factor of calorie burn is how fast your lungs can deliver oxygen to the muscles. Most people burn 350-600 calories per hour working out. Someone who is fit and working out vigorously can push this to 700-800. Figures above 800 should be regarded with a great deal of skepticism. As far over 1000 calories? Pah!!
Also, with an elliptical, it doesn't really matter what your heart rate is - it takes a fixed amount of work to overcome the resistance of the machine. Two people using the same settings on an elliptical will burn about the same number of calories, even if their HRM's produce different results.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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