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Thought gets in the way. When competing, we should be trying to do everything automatically, letting muscle memory take over (like, for instance, walking). Some try to make this easier by using the target, or something else like a piece of music, as a mind-clearing "meditation object".
Practicing is different - sometimes. If you practice properly, there will be times when you are deliberately focussing on the element which you are practicing. Say you are correcting a dead loose - you will be concentrating on your loose at first, trying to build up an efficient set of actions that your muscles can "memorise" - this will affect grouping etc. The hard bit with this then comes when you try to slot this in to your other memorised actions. Gradually you will realign your focus from the loose to your competitive state, until you can do it without thinking.
And then, of course, you must practice not thinking!