I think its human nature.
When you dont have to be so accurate you will find you relax more and are therefore more accurate because of it.
When you know you have to hit exactly what you are aiming at tension mounts and you become a little more nervous and accuracy decreases. This leads to steering the ball and not getting the correct release on it.
I will give you the example as in a golf analogy. You stand in the driving range and bomb out dead straight 270 yard drive after 270 yard drive. Now you go out onto the first hole. Tree lined. Most people now tense up and cant hit it straight no matter what.
As you play more on that condition you will begin to trust yourself to make the shot and therefore your accuracy will improve (hopefully)
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Happy go lucky bowler from the UK
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Did someone say tough luck