hi Karen, i think the main problem is that most girls just wear the wrong sized bra!!!
Before my ba i use to wear a 36b and pad it out with chicken fillets. I had never been properly measured, as never really had any boobs i didn’t feel that it really mattered i suppose.
After my ba i went into a 36dd sports bra, still not realising that this was the wrong band size for me. When i finally got measured after my ba, i was only a 34 band size and not a 36
So although i use to wear a 36 i should have been wearing a 34b and my size now after my ba, is a 34e and it fits well.
Hope that helps. You are right, your band size does not change after a ba, but if you were wearing the wrong band size to start with, then once you finally get boobs, it is more important to wear the correct sized band size. x