Take a measuring tape to a 2x4 and it becomes quickly apparent that it does not measure 2" by 4", but only 1 1/2" by 3 1/2". Did you just get cheated out of a 1/2 inch?!?
Well, no. When a 2x4 is first cut from a log, it is called "rough cut lumber" and is still green and wet. The wood must then be dried, during which it shrinks. The lumber is then planed so that all surfaces are smooth and uniform. Only then is the wood considered finished. So while a piece of rough cut is initially cut 2" x 4", by the time it makes it to our shelves the actual dimensions are 1/2 an inch less. You'll notice that the inch symbol is omitted when referring to the nominal size (2x4), but used to denote the actual size (1 1/2" x 3 1/2")!