FNB58 Cable resistance calculation

Can someone tell me why FNB58 calculates the resistance the way it does? From what I see, it takes average current instead of the one measured with cable in. So Vdrop / ((I_baseline + I_cable) / 2), instead of just Vdrop / I_cable.

I am noob when it comes to electronics, but this guy seems like he knows what he's doing and he shares my view: https://www.youtube.com/watch?v=3yhHX96h-jg (skip to like 33:30)