Why does the kitchen balance scale needle not go back to zero?

Answer 1

It means your weighing scale is inaccurate. You can test how inaccurate it is by putting something that you absolutely know the weight of on the scale (such as an unopened butter tub...) If you scale overweighs/underweighs by exactly the same amount each time, you can still use your scale by adjusting your weighing out of ingredients to compensate. However if the scale is off by a random amount each time, you need to replace it. Digital scales are now fairly cheap and very accurate.