For example, if a 160-pound woman with 35% body fat and a 160-pound woman with 20% body fat are both running at a 10:00/mile pace, the treadmill will display the same amount of calories burned. However, the woman with the lower body fat and more muscle mass is actually burning more calories.
Treadmills also don't take your form and running efficiency into account. New runners will usually burn more calories than more experienced runners running the same pace and distance. Why? The beginner's inefficient side to side movement and bouncing up and down expends more energy than the experienced runner's efficient stride.
Some reports suggest that treadmills and other cardio machines actually overestimate calories burned by up to 15% to 20%. So it's important that you take the calories burned readings with a grain of salt. It's fine to use the numbers as a benchmark for your runs, but don't plan on consuming additional calories based on that number. That's an easy way to start gaining weight, despite your exercise efforts.
If you really want to get a better idea of how many calories you're burning during your runs, try using a heart rate monitor. They're usually more accurate than cardio machines.
Also see: Frequently-Asked Questions About Running and Weight Loss
How Many Calories Does Running Burn?
How Many Calories Do I Need to Burn to Lose One Pound?
7 Commonly-Asked Questions About Treadmill Running