One of the most important aspects of your approach in bowling is to maintain a steady, level, nonmoving head. If you move your head two things will happen.
(1) You will lose sight of your target. If your head moves up or down, your target (the arrow) will move up and down and it will be that much more difficult to accurately hit that arrow.
(2) If you move your head left or right, the angle that you throw the ball to get to the arrow will change. You may hit the arrow but the angle may have changed so you will not hit the pocket.
To the left are some graphics that will illustrate this point.
The very left one shows the bowlers head to the left. The ball misses the pocket to the right.
The middle one shows the bowlers head lined up with his target and the ball hits the pocket.
The far right one shows the bowlers head a little to the right. The ball hits high. His feet never changed but his head did there by making the mark appear to move which in turn caused him to roll the ball at a different angle.
If you are hitting your mark and the ball is not going where you think it should, possibly after you initially line up with your target, as you do your approach your head is moving right or left which will change the trajectory of the shot.
A simple illustration to do yourself.
Hold your finger up to an object a few feet in front of you and line your finger up dead center on that object. Now without moving your finger, move your head a few inches to the left and then a few inches to the right. Do you see how that object appeared to move? If that was your mark and you were rolling your bowling ball you would get a different finishing point (the point where your ball hits the pins) with your bowling ball. It is extremely important to keep your head as steady as possible through your approach and swing. If you move your head as you roll the ball the target will move and your shot will not be true.