Instinctive works better when there are no conscious thoughts during the draw and release. Therefore, the brain is fully receptive to physical reactions and sensations from the body. Any cognitive thoughts, such as calculating an aiming point or causing the arrow point to be placed on that aiming point, reduces the capability of the brain to be receptive to physical inputs from the body. So it is a give and take proposition: how much do you gain by having your brain fully engaged with your body, and how much do you lose by not engaging the analytical capability of the brain? Can't have your cake and eat it too....
It makes sense to me that since an entirely different type of thinking is involved in string walking vs instinctive shooting, the results would also differ. Differ not just in overall accuracy, but also conditionally, i.e. first shot accuracy, sustained accuracy, etc.