Hardly surprising since penalties are very easy to score if the player hits the ball properly and the keeper doesn't move before the kick (which would trigger a penalty repeat).

There's one rule to score a penalty 100% of times: keep the ball ground level and aim near one of the goal posts base. If the kicker doesn't screw up, then it's a goal, no matter how good and well trained is the keeper: there's just not enough time to go there, that's why the secret is keeping the ball low: you can fight gravity with a strong high or lateral jump, but you can't help gravity by adding more force to the gravity pulling you down. Once this has become automatic, the player only has to keep cold and ignore the keeper, which naturally will do anything to distract him, therefore it's only a matter of making the thing automatic and don't think about it. Mental coaching, which recently has become a thing in football too, can help with this.