Originally Posted by Looigi

Top only coincides with the center when the helmet is level. In both your and your wife's cases, it sounds like you both mount the lights "squarely on top" of the helmets. Since yours is tipped forward compared to hers, the tops falls at a different positions with respect to the brim of helmets. If you mounted them centered on the helmet, it would be in front of the top in your case and behind the top in her case.