It also depends on the surface you are putting it down on. I always put mine on exactly the same tile in the bathroom. Don't put your scale on carpet, grass, your bed, or anything else that is soft and squishy.
+1 I always put my scale on the exact same spot in my room and have been using that spot for 2 years or so. Moving it just 2m or so to the other side of the room, same surface though, gives a different result.

