confused... calories remaining?

Mfp is set up to eat exercise calories back. So basically as you log your workouts, it gives you the extra calories you've burnt by working out, back to eat.
But that's sometimes a slippery slope. Mfp is known to overestimate calories burnt. For example, it could say that you've burnt 250 calories for a 20 min walk, but in reality only half of that was burnt.
Some people use a heart rate monitor to get a more accurate reading, rather then using mfp estimates.