...because it sounds better.

Eve

Hey, I want to make a table that is 20 inches wide, are you saying 10 2x4swill leave me disappointed?
I see this issue all the time in DIY makers of things.
I think we do that stuff to confuse the Russians and French.
And seriously is just doesn't work to say "It hit me like a 1 1/2 by 3 1/2 across the forehead."

There is a lot of information on this topic.

Check these articles... 

Two by four lumber 

Nominal Versus Actual: A History of the 2x4

In the past before mass milling machines it was green and imperfect 2 inches by 4 inches. They dried in sun or in a oven to remove water and bugs but wood shrinks and bends. Then a carpenter shaved and sanded it down from a raw cut so give or take it's smaller now. Probably rounding math error to make $

Easier to say

Probably the same reason we cannot buy a half-gallon of icecream!

Because men named it, and we like to exaggerate size. 

Fred,

I remember marveling at that...And someone once explained it to me, and I promptly forgot it.  Happy Monday!  A

It is an on-going battle in our home:  European husband insists on saying 4 x 2 !!   Our home is 1880's built back when wood was wood and proper measurements too, lol

My question is:

Why do they have braille keyboards on drive-up ATMs?

Rounding?

Oh the mysteries of life ... and how to solve them. 

I wondered about this decades ago...The fact is when you want it..speak the language necessary to get it. 2 x 4 gets it done...that's all I know

drying and planing.

Yes...a question from my youth. And I forgot the answer.

Toothpick. Nothing beats rough sawn and beefy Fred Griffin Tallahassee Real Estate .

Because everything looks bigger than it is!

Maybe by time you had the drywall it becomes a 2 or 4 inch respectively?

Because the metric system never caught on here.