I have my own theory which I haven't verified with any shirtmakers. Firstly, it is done to prevent the left and the right shirt-body shifting 1/4" up- or down-wards thereby ruining the pattern matching. Secondly, since the last button is usually tucked inside your pants, it is done horizontally so that the buttonhole will not be folded or squashed. You will noticed that Zegna Napoli Couture have the second-to-last buttonhole done horizontally for this reason, as that's the very buttonhole tucked under your waistband. Most buttonholes measures 5/8" plus-or-minus 1/16", except in Borrelli when the buttonholes measure only 1/2" even though the buttons are very stacked MOP. However, even though most of these buttonholes are 'wide', shifting of fabric is almost non-existent (I think). So maybe my theory is wrong altogether. Does Mr. Kabbaz's shirts have this feature?