The leg on the upstroke weighs down on the pedal and creates additional work for the leg on the downstroke. A crank/hub power meter ignores that and gives you the effective power combined by both legs.

Lets assume a rider generates 120W on the downstroke and -20W on the backstroke. Total power would be 100W.

Now can the crank arm measure the negative power on the backstroke and substract it?

(If, as I've learned, it can not differenciate between positive and negative power a crank arm power meter would show 140W. If it can only measure one side you get 120W)