the measuring instrument is not necessary

The answer to this question is given below in the explanation section.

Explanation:

The zero error of the measuring instrument is not necessary. Because when you measure the difference between two items or objects, you don't need an absolute value.

For example, if you measure the temperature of today and compare it with the previous day, then zero error is not necessary. Because you can produce results either hot or cold. The difference is change.

Another example, when you measure your height and compare it with your friend, you can be either taller or smaller, so sometimes zero error of the measuring error is not necessary.