The measurements are taken by the suppliers themselves and these are the specs they provide to us. Nape measurements are generally taken from the hair at the physical nape at the interior back of the wig down to the bottom of the hair.
The nape is a much better measurement for determining overall length than the crown measurement. Many styles are layered, meaning that they will have short crown measurements due to the cut even if they are long styles. Stick with the nape as your primary guide to avoid confusion.