sizes????

I am wondering if there is anyone out there that knows why plumbing pipe sizes are so confusing. Now lets see do we measure the inside diameter or the outside diameter. Oh galvanized is inside usually I think, but if you actually measure it or look up the specifications it's not really 1/2, 3/4 inch or what ever. It is somewhat different, so 1/2 inch is actually 1/2 inch more or less. How about copper, well sometimes it's inside measured but sometimes you measure the outside. It also seems to be a more or less thing as far as the actuall size.. We won't even delve into the "schedule" issue here. ahhhhh. Maybe that why the plumbers get the big bucks!