scales
It is hard to get good accurate scales for your home. They are delicate instruments that must be tested and calibrated in order to perform consistently.
First, you need to pick just one scale and stick with it for all your measurements. Home scales are seldom dead on accurate, so using a single scale will give you better results than using two different scales.
Second, pick a hard, level surface where you are going to place the scale and leave it there. Do not move it. Moving a scale can cause it to change calibration and you don't want that.
Third, weigh yourself on it three times in a row. Step on, step off. Wait until the dial stops moving, then step on and step off. And again. Do you get the same measurement all three times? If not, there is a problem. It might be that you are placing your feet in different positions, or leaning forward or back. Your weight must be centered over the scale to produce a consistent result.
Finally, try weighing yourself alone, and then again holding an object of known weight, like a bowling ball or a full bag of cat food. Repeat this for three results, like you did above. Did you get a consistent result, and did the higher weight actually equal your weight plus the known weight of the object you were holding?
If these tests provide widely varying results on your scales, try reading the instruction manual for help in calibrating the scale. If you still have problems, you might call the manufacturer or get a new scale altogether. Good luck!