I've had this same problem for a long while now
It seems to affect users randomly! The "Input" calories changes appropriately, but the Burned doesn't on the Overview tab no matter what category I hit, and I do definitely exercise at different levels on different days/weeks (and when I do the averages myself on a trusty calculator, the numbers come out different).
It's weird. I reported it a couple of months ago in here. Not a game-breaker for me, but definitely a little frustrating.
Female, 28 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/3/12 = 164.9 (net: -50.3 lbs.)
Current mini-goal: 160 lbs.
Next mini-goal: 150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."