The timber sizes are different because you're buying planed dried timber. For some reason it was decided that measuring the timber wet and unplaned was a good idea.
On the other hand over here in metric-land if you buy a 44-by-88 piece of wood it'll be real close to 44x88mm, Whether you're buying it planed or not.
That's true, but they had to set their saws to something and a typical sawmill will process the logs as they're brought in, not after drying them because it would take forever to dry the whole log and it would likely split or be infested with creatures. So the logs are sawn 'green' to a rough size, dried (sometimes kiln dried to speed up the drying process) and then planed. Both drying and planing affect the dimensions.
It's quite amazing how the speed of the drying process can affect the later tendency of the wood to warp, as well as the degree to which it will warp. Counteracting this is quite tricky and requires all kinds of techniques, the most effective of which is laminating odd/even stacks of wood.
The same limitations hold in metric-land, and they still manage to label wood the size it actually is when you buy it. I'm sure it's a difficult problem but as far as I can tell it's also a solved problem. I imagine that the odd piece of lumber they have that dries with a severe warp or size deformity simply gets resawn.
Well, in 'metric-land' they don't have a history of naming their wood for the thickness of some guys thumb in integer multiples.
The USA could switch from 2x6 to 1.5x5.5 (planing takes 1/4" off each surface) tomorrow if they wanted to but I guess there would be quite a bit of confusion. The building industry is super traditional.
And when you do that you might as well go metric (for reference: 38x140mm).
> in 'metric-land' they don't have a history of naming their wood for the thickness of some guys thumb in integer multiples.
Yes we do. The metric system didn't fall out of a clear sky one day, it had precursors.
That's hardly the point though, even when using metric you could make the decision to measure wood wet. Even when using the imperial system you can make the decision to measure wood dry.
Everybody thinks that, but it isn't true. historically some sawmills where producing 2x4s that were 2x4, while others were using different sizes (I can't find evidence but I'm guessing they were trying to cheat their customers for profit and get away with it).
What matters isn't the actual size though, what matters is that when you are short a piece of wood you can get one that fits.
On the other hand over here in metric-land if you buy a 44-by-88 piece of wood it'll be real close to 44x88mm, Whether you're buying it planed or not.