I've been having the same problem for a few weeks now
It doesn't change the number if I look at week, two weeks, past month, etc. -- it just shows whatever I did today (on the overview tab). I'm pretty sure this is a bug.
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."