The problem arises from two factors, the reading conventions of English, and the human visual system.
English text is read from top to bottom, and from left to right.
Thus, the most important part of a page is the top left. That's where people's eyes go first. The voters looked at the top left of that ballot page, and saw a set of names down the left side of the ballot. Visually these look like a group, because of similarity and proximity, so the visual system makes them into a group. A voter would now have processed a column of names, and a column of holes.