Playing a deep line usually means having to "open up" the shoulders, and that isn't what you sound used to.
If you consistently play an outer line, or any line that requires squaring up, you develop what is called "muscle memory", meaning your body begins to memorize the motions that result in success.
Then, when you try to do something your body doesn't recognize ( like open your shoulders and play a swing shot ), it wants to continue to do what "feels" correct, resulting in you subconciously trying to square up your shoulders and pulling the ball inside your actally intended target line.
Something that often happens to me as well, is timing. If my feet get fast, and I get to the line before my armswing is ready, I muscle the swing with my shoulder and miss my target to the inside, effectively causing a "pulled" delivery that misses my target to the inside.
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"The difference between stupidity and genius is that genius has its limits."
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