This happens on mine if I don't let the scale calibrate before the first measure. The scale turns on if you step on it, so it's tempting to just put both feet on in quick succession while the scale it's still off. That will give you a wrong reading. If you step off, the scale calibrates, and the next reading will be right.

You should get two equal readings if for the first one you push on the scale with one feet, take it off, wait a few seconds, then weigh yourself.