I've been wondering about this too. I feel average, or slightly above, but clothing manufacturers seem to think I'm big, because tops tend to be baggy through the shoulders. Yet my shoulders are not particularly narrow, being well-balanced with my hips. I definitely don't want to wear a minimizing bra, because that would accentuate my thick waist. Just can't win here.
Since I've been on YLF, I have gotten much more particular about getting things that fit right, and it's not an easy task. At least it helps prevent a lot of impulse buying. ;o)
As for sizing being inconsistent, I can attest to that. My two newest bras were 36D (band definitely not tight) and a 34DD (cup definitely not tight), bought at the same time with the same SA. She assured me that that DD cup in that particular model was smaller than normal. So much for standard sizing. I was feeling pretty much like Shiny, exhausted and wanting to just go home, but I really, really needed some T-shirt bras.