Why is a 2x4 called a 2x4 when it is actually 1.5 x 3.5?
2X4When It Is Sawed In A Saw Mill It Is 2" X 4" , Then It Is Kiln Dried And Surfaced. Therefor You Loose Material, Most All Wood Sizes Are Given In Mill Size. 2X6, 2X8, 2x10 ect, These Are The Sizes They Were Cut At The Sawmill.
2x4's used to actually be 2"x4" but the timber companies realized they could cut them down 1/2" on each side without losing any structual integrity and therefore they could get more useable wood out of a tree. I used to frame houses and now I re-hab them. If you ever tear the drywall or plaster out of an old house the studs will measure 2"x4" not 1 1/2" x 3 1/2". The finishing process mentioned above may reduce the final product but is not the "reason" why they are smaller today. If they wanted to make a true 2x4 all they would have to do is make the rough cut 2 1/2 x 4 1/2 and end up with a true 2x4. It was purely a cost saving move.