It was a mistake born form Geoge Lucas’s inexperiance with fight scene at the time, quite why he never thought to adress it as one of the many chnages he made in his SE I can’t fathom.
What?! George has said he doesn’t like human violence. That’s why Obi-Wan didn’t go around cutting down Stormtroopers. I wager he deliberately went with the severed arm that would be the most alien.