We're not talking about measuring a body; we're measuring a piece of clothing designed to fit a body. You can take a piece of wrapping paper and wrap it around a box, and then flatten it out and measure it. It doesn't mean the package was flat.
The garment is not the exact size of the chest or it would be tight to the skin. It needs some amount of looseness for comfort and drape. The armpit to armpit method works great, unless the person is pregnant or has an extremely large belly. Then you will take the back measurement plus from the armpit to the buttonhole and from the other armpit to the button, and do the same thing at waist level. This kind of measurement is used in suits labeled "portly."