Not made up. It’s based on the old green vs dried lumber dimensions. The green wood was cut to 1x4" and it would dry to something less. Most all commercially sold wood is now kiln dried and ends up with a pretty standard size - not many people build with green wood.
The made up stuff is plywood. It used to be that half inch plywood was half an inch thick. Now it’s 15/32 or less. Since ply is manufactured, they slice the veneers a little but there’s no drying or other post-manufacture process changes that affect its thickness, just extra profits from every 15 sheets. 