It couldn't be more simple: The instructions for making shots and positions are written into your brain nicely, but all the CROSS CHATTER in your brain at the time you want to perform makes a mess of accessing those instructions without static.
One simple example is self-doubt: You all of a sudden QUESTION your ability to make a shot that you have in fact trained yourself to reliably make. Your brain then says to the area that holds the instructions "No, those AREN'T right!" So it modifies them or ignores them.
One solution: During practice, "make believe" each shot is under enormous pressure--the big make-or-break shot of the match. Do it in a way that's real--you have to conjure up all the feelings as best you can. Then, you'll be able to shoot at a match in just the way you shoot in practice, because the experience won't be a distraction. Furthermore, you will deaden the emotional novelty of "make-or-break-match-shots" through the process called "extinction."
In general: If you can't have the same mental state during competition that you have during practice, then you won't perform the same. Since it's generally IMPOSSIBLE to have a "zoned out," fearless, unemotional state during competition, it's almost useless to allow yourself to have that state during practice--or else you'll always have the experience of being WAY better in practice than in competition. DON'T "play in the zone/play unconsiously" in practice, so that you don't require that mental state when you're competing.
Edit: That's why it's good to go over fundamentals, and have every aspect of your play well-defined and conscious. Then, you can always depend on that in any situation "To make this shot and get this position, I do X and Y and Z, and then A then B." If you train your mind to feel and know THAT is the basis of what you do, then you can always simply do that any time you wish. On the contrary, if everything is just a "feeling" or "mood," you can't bring those things up at will--and so there will be times when you can do it, and times when you can't.
