Not to worry! If the difference is 1 or less (this includes negative numbers) it just means you fall within the AA cup letter.
For example, if you measured 34'' around your bust, and got 36 band size:
34 (bust) - 36 (band) = difference of -2 inches, for a 36AA in Pepper.
|Difference||1 inch or less||2 inches||3 inches|