Why do Pocky and other "stick" biscuits have this distinct burn pattern?
While I cannot give an absolutely definitive answer, from visual observation of the picture you provided, that is simply the pattern that would develop when a small cylinder lies across parallel wires of a grill at an angle.  The contact surface is an ellipse where the biscuit was in contact with the grill, the width and length of which were determined by the angle at which the biscuit was oriented.

The biscuits are probably cooked twice, once to cook through and be strong enough for the second baking, and once to crisp up.  The markings would be a result for the second baking.

The one or two exceptional sticks with doubled markings probably rolled or shifted slightly during the baking.