Unless you can enter your weight and age into the cardio machine, all they give you is an "average". And even if they do have the settings for you to enter those, I still think it's not completely accurate. I think the manufacturers make them over-estimate the calories burned, so you are motivated and keep using them.
Here's a site that can also give you an idea of calories burned --->
Calories Burned Calculator. Compare what the cardio machine tells you, to what this site says, and you may have a better idea.