Hi folks.
If the object being measured is not exactly up to the precision of what you specified in your Units, you will get this ~ (approx.) symbol.
For example, if you specify 0.0 mm and the object measures 3.001 mm, you will get this ~ symbol. Just push the precision to more decimals (3 in this case).
Even if you did not enter such numbers, dividing a line or generating curves, for example, can lead to such numbers.
Just ideas.