It’s not. It’s 1.5" x 3.5". I don’t know the history and wasn’t able to find it in a quick search, but lumber sizes are usually a half inch less than the name would imply.
That only applies to the thickness and width, though. The length of a board should be as described (e.g. a “two by four by eight” would be eight feet long, but have a cross section of 1.5 by 3.5 inches)
It’s funny that you used 2x4-8 as your example for length, because a 2x4-8 stud is not actually 8 foot. If you’re looking for 8 foot 2x4, you need to be careful that you are buying true 8 foot boards, and not 8 foot studs.
That being said, that’s the only board I’m familiar with that has a length that’s not always true length
The story is that the wood used to be roughly 2x4 and by “roughly” I mean that the woodworker would have to throw away almost half an inch to even have a straight surface.
But when the machines to do that became common the mills started doing that work, and they saved the extra volume of essentially garbage that was shipped with every piece of wood.
