Most people drop some water weight at the start of a diet, so that might contribute to what you are seeing. I'm sure it's not all water, though. How long have you been on your diet?
That is a great start,and a really good question. Im no expert , but I agree with the post above me. I think it is water weight. I heard the first 5-10 pounds is water weight then anything after that is body fat.
Is your scale reporting a percentage of fat or pounds of fat? If it's reporting a percentage of fat, but your total weight is down, then your actual body fat *has* gone down.
I've been doing WW for a month now almost to the day. I have a *very* hard time believing that my 7 lbs is nothing but water. My scale gives % body fat and % water, and neither has gone down, but my weight has. I don't know. My hubby has lost 10-15 lbs and his % body fat hasn't changed either according to the scale. *shrugs*
Are you exercising? If not, it's possible for your body fat percentage to stay the same or even go up even though you're losing fat (but losing more muscle)
Also, I wonder what the margin of error is on your machine. A seven pound loss may not be enough to consistently reflect a downward trend.
Unfortunately those scales are not always accurate. As long as you are exercising and losing, you are doing the right thing so don't let it get to you.