Lumber shrinks, but not that much. There was NO standard and so mills just did what felt they could get away with calling a 2x4. Some mills did 2x4 was 2"x4", some did other sizes, it depending on how much they felt they could cheat vs how much they felt advertising the larger size would help. I have seen houses built in 1880 that used modern dimensions for 2x4s.
They claim that the change was driven by railroad shipping charges, and wasn't based on drying, but on pre-planing the rough lumber to reduce shipping cost. They further claim that in 1919 the US Dept. of Agriculture studied the issue and ended up defining a national standard for what the post-planed dimensions of a 2x4 should be. And they further claim that it took until the early 1960s to settle on a new standard that matches what we use today.
I've seen houses built in the late 1960s that used non-standard 2x4s, so I will dispute those facts. I don't remember details (and can't be bothered to look them up), but IIRC several different standards were tried before the current one finally took.
The pre-planing is a common claim, but I don't believe it. They can make lumber whatever size they want - of course they need to plane it, but they just make it larger to account for that. Still the planing excuse it one they like to use because it doesn't show "them" as trying to cheat us.
