Although I suspect this particular quirk of dimensional lumber stems from the British, the result is not too unexpected for modern-day America. After all, we (insanely) deal with sales tax the same way, where the advertised price is pre-tax, and consumers have to do math if they want to compute the final bill before reaching the checkstand.

So having to measure the lumber to acquire its actual dimensions is entire above-board [pun intended] for anything beyond putting together a wood-frame structure.