I think it is general knowledge that a 2×4 does not measure 2 inches by 4 inches. They are usually about 1 1/2″ by 3 1/2″. The length is not supposed to vary, but often does. I have found 8 ft 2x4s vary by as much as 3/4″. Usually they are too long, but sometimes they are too short. Assume they are correct, and you may find your project doesn’t quite fit together.