We had some electric scales that would read 2-3kg difference every time you stood on then.
Apparently the tolerance can be quite wide as you are using 4 load cells (?) each of which has a tolerance which gets aggregated. Presumably temperature or voltage could put them all towards one end of the tolerance.
So I bought some better scales (body mass measuring ones) and they gave a rock solid weight each time like the OP, but as someone else suggested, I think they just remembered the first weight. Annoying when you want to hold the cat to weigh it!
They’ve broken now so thinking about getting some old fashioned dial ones, don’t really like the glass top all the Digi ones come with as I tend to stand it against the wall in a tiled bathroom.