[bombela]

https://en.m.wikipedia.org/wiki/Lumber

And what's is worse is how hard it is to know the true size of everything you find in stores in the USA. The same applies to pipes of all types. But even things like a drawer slide is often not the advertised length. Saw blades thinkness is converted from 2.5mm to whatever the closest inch the salesperson felt that day.

From having spent quite a lot of time in big box stores in France and the USA. I can tell you that in the USA 3/4" (19mm) can be anything from 16mm to 24mm.

In short almost everything is built in metric and the USA likes to pretend it's not and gets confused by rounding up randomly.