occasionally, I've found that depending upon how I stand on the scale can affect the weight by a few pounds. For example, putting the weight up near my toes, back on my heels, or evenly distributed. Its probably not like that for every scale, but my digital one has the sensor pads in contact with the floor but only where your heels are. The fronts pads are merely pads and sort of act as a hinge which throws the weight off I believe if you don't stand on it right. Also make sure your body touches nothing when weighing .Don't grab anything for balance for example and expect an accurate weight. The weight relieved by resting your hand on a counter can be more than you think, people arms can weigh 10-30lbs.
Other things: The cloths you wear can affect your weight(I'd suggest when weighing just hop on it each morning when you get out of the shower naked). Women also vary weight much more than men especcially during the TOM(but even outside that).