Unnatural arm position is a factor to be taken into consideration depending on the context. It's not the be all and end all.Unnatural arm position is the keyword I’ve heard to make such judgement.
Throwing his body with tuck arms to block the shots that hits within a second is not a clear and obvious penalty.
Tarkowski moved towards the ball and then blocked it with his arm. Natural position or not is pretty much irrelevant in that situation.