I wear a Polar monitor which conveys my heart rate to a Polar compatible aerobic machine. I go by the machine calorie burn.
Today, I didn't wear my monitor, but warmed up for 10 minutes on the elliptical prior to my strength training. I entered my weight, age, etc into the machine and it recorded my heart rate. The machine indicated I burned 51 calories. However, Spark says I burned 70 something calories.
I read a while ago, that regarding calorie burn, some home machines are calibrated for 150 pound people. Maybe your manual will tell you what it is.