The answer is BREASTS! Yepee!!
Even at an A cup that's a lot more breast than one inch bigger than the chest measurement. There needs to be a starting point for the cup letter system to work. That system is based on a one inch difference between each cup size. You have to have a starting point for the letters to work. So, by having whatever the bra maker needs to get to their starting point is why all the various add's to the band size. Bra's are a lot like shoes, its amazing how your size changes depending on who made it.
So, the answer to the question is that the additional inches is to give a starting point to where the cup system will work in sizing. Yes, there are a lot of other ways that could be done, but this is the way that the industry has chosen to use.
-- Bad Irving