All bathroom scales are witchcraft.<p>Analog ones always seem to have the precision of ±10kg and generate an aura of untrustworthiness around themselves. Digital ones behave as if they were haunted. Every once in a while, usually when the upward-opening crescent Moon passes through an astral node on the sky, my digital scale just picks one of two or three values at random. The values are around 1-2kg apart. I learned to just step on and off it until it either settles on a single result, or I get tired and just end up averaging last three or five measurements.