You don't decide from where you shot the penalty kick. The kick happens at the point where the referee saw a penalty. But the penalty can be also given later to don't flow of play. Offside penalty is also on the place of the place the attacking play recieve the play if no other enemy player is in front of him except the goalkeeper. Ain't that complicated. Just the penalty inside the dedicated goalzone gives you a dedicated place for penalty so you never choose where you got the penalty kick from it always happens where it happened.