Depending on what type of scale you have and its quality, moving your scale can throw off its balance mechanism, just like moving a piano can throw it out of tune. Try resetting it to 0 (zeroing it out) after you move it, that should help.
However, it could just be that your scale just isn't that consistent. I had one of those once. One of the tests for buying a good scale is consistency, so move it around and step on and off a few times. A good scale will give you the same reading each time.
But if you think your scale is a decent one and zeroing it out doesn't help, or you just don't want to buy a new one, I would suggest not worrying about the specific number quite so much. Yes, it's great to be able to say "Hey, I lost 4.7 pounds!" but really, does the actual number matter all that much? Just about any scale, unless it's *really* out of whack, can tell you if you're going up or down. If it says 251-255 today and it says 240-243 a couple of weeks from now, chances are you're losing weight.
Besides that, you know if you're doing well no matter what the scale says. There will be scale fluctuations on a daily basis whether you move the scale or not, and I, as an admitted scale addict, can personally attest to that.
Lots of things can affect your weight at any given moment--water, sodium, glucose, the food still in your stomach, the last time you used the bathroom, etc. But you know if you've been eating healthily, exercising, and drinking your water. If you're being healthy, that's what counts, not some silly little number.
Hope this helps!
~Elisha