When you press down on to the sink you are applying a force to the sink. This force includes some of your own weight. So you are applying less of your weight to the scales. This means that it will say that you "weigh less".
Basically, scales are dumb. They don't measure how much you weigh. They just measure how much force is applied to them and turn this into a mass/ weight. It will only give you your true mass if you make sure that you are perfectly stationary, applying only a force through your feet to the scales. Oh, and naked, as clothes can themselves weight something, although in practice people usually don't fuss about this for some reason.
Baldric, I'd say it was because you were moving, which means that there could be another force to be considered. If you'd gone back to standing still and hovered on one foot with the other not touching the ground it would have gone back to giving your full body weight.