Builders merchant lengths ...
3.1, 3.2, 4.0, 4.4 .. I think maybe to do with the most efficient lengths from the sawmills and machine shops.
Best to use longest lengths where you can. Keep cut lengths out of sight and use end grain sealer where it is cut or butt jointed.
AlBaggo is obviously far to young to remember .............. it's the equivalent of the old Imperial sizes .................. so that the changeover to metric didn't frighten the building trade too much lol