I'm guessing that the 2x4 thing is similar to how all soda is called "coke" in some areas and how all in-line skates are called "rollerblades" (I used to skate for them and they faught this as best they could!!) and how all cotton swabs are called "q-tips" etc.... Older homes DO have 2"x4" 2x4's. I am guessing that there was a gradual change to the new size and that carpenders still refered to them as 2x4's because they used them inplace of what the original 2"x4" versions were used for....

But that is pure guess.