A quick Google search says that about 75% of penalty kicks are successful. If you're telling me many people don't even try to put it in a place where the goalie can't get it, yet 3/4 of them are still going in... I don't know; that just sounds like there is an awful lot of guesswork involved.
It's all guesswork from the goalies point of view. What I am saying is that if the person taking the penalty does it well it makes no difference what so ever if the goalie guesses right or not. But even if the taker is trying to put the ball in a spot like that, the success rate isn't 100% because sometimes the ball doesn't actually go exactly where the taker wants it to go. Sometimes instead of the ball being a foot inside the post it's a foot outside the post. Sometimes instead of the ball being a foot or two under the crossbar the ball goes over the bar. Those misses don't have anything at all to do with guess work, they have to do with players not being machines who can place the ball exactly where they want to 100% of the time.
And sometimes the taker wants to put the ball a foot inside the post and instead they hit it five feet inside the post, and in that case whether or not the goalie guessed right is really, really important.