That’s another oddball. From what I have understood ”two by four” lumber in US about half inch undersize.Worked in the Construction Industry in Canada for 4 decades & didn’t really go to the lumber store for 38x89s or buy 18mm 1220x2440 sheets of plywood ,,, but we “wood” joke about it .
Here it is also commonly called ”two by four” even if we never ever use inches in construction. But unlike in US it is lot closer to full 2”x4” as dimensions are 51x101mm +-2mm or more commonly 48x98 +-1mm