It's possible that if you're exercising, your body could be getting smaller, even if the scale isn't moving.
I have to clarify CRSeaside's comment.... muscle doesn't really weigh more than fat. A pound is a pound, no matter what it's made of.
I, typically, compare muscle and fat, with rocks and feathers. The molecules of a rock are packed so densely, that a 1 pound rock is usually not going to be very large.