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.

There is the need of two different names for your underbust and overbust measurement to get the perfect size. For example, two women might have the exact same underbust measurement, let's say 34, if one of them have A cup breasts, 34A is her bra size, if the other has B cup breasts, then her bra size is 34B. The system works perfectly fine.

Although very rare, some brands only work with the underbust measurement (e.g. just "34") and set the cup size to the average size, so those who have bigger/smaller breasts than the average for their weight can't find a bra from those brands.