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, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/29/14 = 180.2 (net: -35.0)
Current mini-goal: 180 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."