@BalrogDeMorgoth Cup sizes aren't an exact measurement of breast volume; rather, they're a ratio of breast-volume-to-ribcage-circumference and are therefore relative to the band size (ribcage circumference). Bra fitters will often recommend "sister sizes" if they don't have the exact size in stock—32C and 34B, for instance, have different cup
sizes but the same cup
volume, and so a "sister size" may fit someone better in a particular line of bras because of differences in cut. Like other types of form-fitting clothing (e.g. most women's clothes) a lot of human variation is distilled down to a handful of sizes. Your "size" really tends to be a point you start from when figuring out which size to buy from a given brand (or line of clothing
within a brand) rather than a real, concrete value, because oftentimes a size that will fit you in one brand will fit poorly in another.
I recently measured myself from scratch to find out where I should start in replacing my now-too-small bras. Six different measurements punched into a calculator, plus evaluating whether my breasts are top-heavy/bottom-heavy, wide-set, shallow/projected, and so on all go into finding the right fit if you want to minimize the trial-and-error aspect and ensure you don't end up settling for something not-quite-right. (In the end, I
still ended up buying bras that were in a different size than the one recommended by the calculator.)
If you or anybody else wants to read about any of this in more detail,
here's the guide I used.