SparkPeople’s database estimates the number of calories you burn during exercise, based on generally-accepted rates of calories burned for various activities. Our tracker also bases your individual calorie burn on your weight, because people of different weights burn calories at different rates. (A person who weighs more will burn more calories doing the same activity as a person who weighs less.)
Cardio machines are also estimating your calorie burn. Some can be more accurate than others—for example, if you input variables like your age, height, weight, and gender. However, even the actual machine you use tends to be inaccurate—and some overestimate calories burned by up to 30%.
Both SparkPeople's cardio tracker and cardio machines estimate the number of calories you burn. Estimates are always a problem because you never really know how hard a person is working. For example, two different people could go on a 2-mile walk at a speed of 4 mph, and that could be very easy for a fit person (so they’d burn fewer calories), yet very difficult for a beginner (so they’d burn more calories). Because these exercise trackers don’t really know how hard you are working, the estimates can be a little off.
The best way to monitor calorie burn is to wear a good heart rate monitor. This works better because your heart rate is an accurate measure of how hard you are working, and is directly related to oxygen consumption during exercise (which is the true predictor of calories burned, but you’d have to visit an exercise lab and be hooked up to a machine for that kind of accuracy).