the other prob is, that when yo measure the fullest part of boobs, it could be say 39, but one person's boobs might be 'longer, or fuller' than the nexts. So when you gather them up and put them into a bra there mgiht be more volume.
We just have to find a cup that the boob wil fit into, coupled with a comfortable fit around the chest.
Avoid stretchy bras, stretchy thin straps. Too much 'give' in the material means less support, more movement. less comfort.