being picky to think that a 2"x4" should actually be THAT - not 1 1/2"x3 1/2".
And that is the crux of the problem. A 2 by 4 used to be 2" by 4". Then someone decided they wanted to plane it down smooth (that was rough cut) so they planed it down to 1 3/4" by 3 3/4". Then some marketing person decided if you could plane it to that, why not cheat it down a bit in size and then plane it to something even smaller. I was a carpenter for many years and I've seen "dimension" lumber that varied much over the years. The older it is, the thicker it actually is. There was a time that a 1 by 10 was 9 5/8". But it's not anymore. A lot of the commercial companies in recent years have shrunk their deeps down to 9 1/2" and a few are even 9 3/8". If you intend to make all your own hives, you can just use an full width board for your box and make your frames so that the beespace is right (basically the frame should be 3/8" less than the box in depth) and it will work. But if you sold it, it's a nonstandard size and that will make it not worth as much.
It actually works out well for me often, since I do all mediums I don't have to waste as much ripping it and when I want a bottom attached (for swarm catching boxes or nucs or long hives) I don't rip it at all and I have the correct space at the bottom for standard medium frames.
When I was using deeps and I made any of my own equipment, I used the ripped off pieces for cleats for handles, and for cleats for migratory lids and for making screened bottom boards. They didn't go to waste, but it's still a waste to have to by a one by twelve at a higher cost per board foot than a one by ten.