Why am I wearing a B cup?

According to my measurements (bra size subtracted from band size) I should be an A cup. I'm wearing a 36 B, and I guess I'm simply wondering why it's not an A. Is there a purpose a woman would need to progress up a cup size like that?Why am I wearing a B cup?
Different brands different sizes. There is no norm for bra sizes so you could be a 36 A surrounded by one brand of bra and a 36 B in another brand of bra.
bras are a one size fits none
merely ware what ever size fits right
Hormonal fluctuation, Different brands of bras have different varying sizes. Just wear and work near what fits.
Coming from a women heath physician most women are contained by the wrong bra size if you want help on finding the best one you probably should hold for your health in recent times email me. doc