For example, most machines don't take into account your body fat percentage. A person who has a higher percentage of body fat will usually burn less calories than a person with more muscle mass. It also doesn't take into account your fitness level...if you're new to an activity, you'll usually burn more calories than a fit person doing the same activity.
Some experts estimate that cardio machines can overestimate calories burned by up to 10-15% so it's best not to take these numbers as gospel but, instead, simply use them as a benchmark from workout to workout.
Instead of relying on calories burned, go by your perceived exertion and/or keep track of your target heart rate. If two machines show the same number of calories burned but one feels easier and keeps your heart rate lower, that's a sign that you're probably burning less calories than you think.