OK, a scale is just a machine. Depending on how old it is, what kind of machine it is, it can do just what you described. That's why the general rule is, get on it ONCE and call it done. Otherwise you'll be on and off it all day, trying to get a "better number."
It's generally best, if you weigh once a day, to always weigh at the same time, and wearing the same clothes or no clothes. I weigh in the morning after the bathroom, because that's when I weigh the least.
Jay