Why is a 2x4 called a 2x4 when it is not?

The "rough size" of a 2x4 used to be 2" x 4" and they run it through a machine called a planer, which takes 1/4" off all four sides. Today they have better planers that don't need to take off that much wood, which allows them to get more 2x4s out of a tree, but that's why they're 1-1/2" x 3-1/2".