For me, it's a natural consequence of filling a box line-wise vertically from top to bottom. If you want to use the sheet layout variant with 3 columns and boxe

For me, it's a natural consequence of filling a box line-wise vertically from top to bottom. If you want to use the sheet layout variant with 3 columns and boxes of variable height rather than a fixed 3x3 layout, the vertical scheme seems again more natural. But whoever feels more comfortable with a horizontal scheme should do so.