As a P.G.A coach with 20 years experience a common fault I see every day when coaching both elite players & beginners is they swing down the target line instead of back inside the target line, some players refer to it as Exit left or Swinging left. You will notice this movement with tour players and in the example picture on the right.
For an elite player, swinging down the target line will cause you to hit it to the right so you then have to compensate and over rotate your hands to try and square up the face, as you can see in the picture on the left, so then you hit it left, so this leads to unpredictable results. If your timing is right you might get away with it, but it is the main reason why the ball won't go where you want it to go.
For beginners & intermediate players swinging down the target line will cause you to chicken wing, disconnecting the arms from the body resulting in poor contact and loss of power & control.
The fact is, if you are a good player, you are better at compensating and getting away with it every time you play because you can still score well and your swing is fairly good, quite often though you hit the ball on the flag but deep down you know you made a compensation because you felt it, so you where lucky that time!
As for the beginner/intermediate player you are not so good at getting away with it, that is why you don't score very well and never improve. It amazes me the length of time people have played golf for 10, 20 or 30+ years and still of a 28 handicap! The fact is that whatever you level you are probably making some compensations in your swing.