The wool does not lie straight when it is lying freely on a surface.
Measure the length of wool under a specified amount of tension to straighten the wool while the length is measured. By applying these same conditions to the measurement of any other lengths of wool a fair comparison can be made. Also by specifying the tension other people are able to make comparative measurements under the same conditions.
How to apply the tension. Using any kind of grip to hold the ends of the wool will prevent that small end sections from stretching when the pressure is applied and it will also be very difficult to measure to the exact ends of the wool.
Tag some string to the end of the wool using a spot of superglue and grip the string when applying the pressure.