maybe this is why
lower transoms are run horizontally so if you run the planking in the same direction, you will get long seams that are not backed up with anything. Like the blue arrows maybe you might get a seam that falls on a transom like the green arrow.
Another problem with horizontal planking is the face of the transoms are not flat they bevel so a tight seam is difficult.
if you run the planking on an angle first you can bend a plank to the beveled face of the transom and if the transoms are placed close together you can nail along the edge of each plank and into the transom.
the upper part is the stern timbers and they run vertical so the planking would run horizonal
View attachment 301087