Different scales can weigh you something different because of calibration and the flooring. You need to use one scale as "official" scale and always do same same time, same scale, same clothes, same everything for the most accurate gauge of weight loss. I prefer in the morning before eating or drinking anything and after going to the bathroom. After that, my scale can go up and down all day long (our water weight). So if you weigh in on a different scale than normal, don't worry too much about it if it says you have a gain.