I've tested my scale by weighing right after driving home from WI with my coach and they are usually different. There is no way I could have gain 0.5lbs in a 15-minute drive so it has to be a variation in the scale. Try it out a few times to see if you can predict what the difference will be. Also if you aren't doing the WI first thing in the morning after a trip to the bathroom and before ingesting anything including water, there are going to be day to day variations from your water or food.
Stress over the scale won't help your progress either. If stepping on the scale causes you frustration between WI then stop stepping on the scale. Trust the program and it will work. If you do use the scale at home, take note of what it says but trust yourself and your actions more than the number it spits out on any given day.