That argument sounds like a stretch. I would’ve thought those unit counts were instead used due to resulting in a convenient, space-efficient rectangular ‘slab’

That argument sounds like a stretch. I would’ve thought those unit counts were instead used due to resulting in a convenient, space-efficient rectangular ‘slab’ size when packaged together: 6 pack = 2x3, 12 pack = 3x4, 18 pack = 3x6, and so on.