Why do Heart Rate Monitors show lower calories burned then the cardio machines?

I recently incorporated the use of a Polar Heart Rate Monitor into my exercise routine. I work out 6 days a week. 5 days consist of 45 minutes of cardio and another 45 minutes of weight training and the 6th day is just the 45 minutes of cardio. On day 7 I rest.

I always love watching the calorie counters on my favorite cardio machines such as the eliptical bikes and cross trainers ... they tend to tell me I burn much more calories in a 45 minutes session (about 575 to 600). A treadmill says I burn about 350 to 400 in the same time.

When I started using the heart rate monitor I noticed the calories burned on the HR monitor showed lower ranges on the eliptical bike & cross trainer ... about 375 for 45 minutes verses what the machine said (575-600). As for the treadmill, the HR monitor was closer in range to the machine.

I'm wondering which one is more accurate (the HR monitor or the machine?) Both devices make you imput your age and weight ... so why are they so different?

The exercise machines are only estimates and they do not take into account how fit you are. The heart rate monitor is probably more accurate because it sounds like you are more fit than most people your age, so your heart rate would be a more accurate reflection of your activity.

Unless you input your weight, the cardio machines at the gym use the weight for an "average" person--150 pounds for the machines at my gym, but I don't know if they all use the same weight. If you weigh more than 150, you are actually using up more calories; if you weigh less, you are burning fewer calories.