HRMs v. Exercise Equipment - Accuracy of Calories Burned?

Just got back from the gym, and wanted to share some learnings about the accuracy of calories burned using HRM's v. Exercise Equipment. I used my Puma HRM for the first time at the gym tonight and was surprised to see the difference in calories burned according to the HRM v. the calories burned displayed on some machines. The biggest difference came through using the Cybex Arc Trainer that reported 946 cals burned in 38 mins v. the HRM that reported only 508 cals burned in the same period. Both devices had my weight programmed in (297 on the HRM and 295 on the Arc), and when I checked my pulse on the Arc it was within a couple of bpm's of the HRM, so the only thing I can attribute the difference to is the constant monitoring of the HRM v. the intermittent monitoring on the machine. Since the pulse check on the Arc isn't constant, I have to assume that the cals burned calculation for the excercise equipment does not factor in pulse at all. However, when I used the LifeFitness treadmill and stepper, both machines automatically picked up my heart rate, and the bpm's were displayed on the machine. These 2 machines had cals burned calculations much closer to my HRM, which suggested that the constant heart rate monitoring was factored into the cals burned unlike the Cybex that used an estimated heart rate for its calculation. I guess my learnings here are two fold - 1. Use the HRM wherever possible for the most accurate cals burned calculations. 2. If you don't have an HRM, you have to factor in your fitness and discount the cals burned. In my case, though I weigh nearly 300 lbs, my resting heart rate is in the 50's, and I find I need to crank my effort up to get into the 120's - so, unfortunately the internal calculations used on many machines will exaggerate my cals burned. If anyone has any other insight on the subject, please share!