Some methods or techniques for taking measurements are better then others.

The image below shows a poor method for finding the diameter of a circular coin. It is poor because you cannot exactly see where the edge of the coin meets to ruler:

In the case above, the measuring tools have been used poorly so the measured value will not be close to the true value.

As an improved method:

Using set squares, we can line the edges up much better with the coin so you will get an improved measured value. For this reason, we must distinguish between the measured value and the true value.

While the true value is a constant, the measured value may change depending on the way the measurement was taken.