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, 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."