what's the deal with LFW lengths

okay so my Rex i have is 20". The length of the hair from root to tip is actually 20". i ordered a 20" lfw for my cousin from the same vendor i received mine. The actual length of her hair from root to tip is about 14-15.5". But if you measure it from the crown of the wig to the bottom of the wig it's about 22". Is length measured differently with each vendor? my wig was a custom and hers was a stock. But they were both requested at 20". what's the deal on these different lengths?

I find most tend to be spot on or a bit generous.  But considering how there is an art/science to construction, they are bound to vacillate here and there. I notice some of mine have graduated layers.