Why do so many women wear the wrong bra size?

Bra size isn't just letters. AA, A, B, C... are the cup size (aka breast size), there is also the number for your "band size", the underbust measurement.

How does a lingerie store determine what quantity of each bra size to carry?

 Generally speaking, the optimal amount of stock that is kept on hand at any given time is based upon historical sales.  Of course, the heaviest inventory levels would go to the most popular-selling sizes, progressively decreasing in size in unison with sales.