Assuming a perfect scale and a perfectly still person, you should weigh the same amount on one or two feet. However, I imagine that you are slightly off balance when you try to stand on one foot. As a result, you are putting more force onto the balance as you adjust your weight and try to stand still without falling over. This force pushes against the balance, which makes it appear as though your weight has increased.

An alternative explanation is that you have a poorly-designed balance, and if you have too much pressure (that is, more force over a smaller area), it distorts the measurement.