It could be static electricity some thing that can and will make both a electronic and balance beam scale jump a little or even give a poor reading. Do you have any Styrofoam near your scale? Even the smallest air flow can effect a scale.
It also could be a weak battery.
My worst scale ever was a lyman balance beam. I could never get the scale to balance. I had yo go out and buy a new scale. After several years I notice the problem with my lyman scale. A very small tab had broken off of the beam near the pan holder.
I never could figure out how that broke but once that happens you just can't zero it,
. I can't tell you how many hours I had into trying to balance that scale for about a year. I was thankful since it would not balance I never used it of course.