It's also not practical to fit into a size 32 if one's true band size is 40, any more than someone with a 40 inch waist can wear size 32 pants.

You can go the other way more easily and in my understanding/experience that's typically what happens (i.e. someone who's true size is 32DD wearing a 40A due to a limited set of options available in many brick-and-mortar stores).

Picture trying to fit an orange into a cup that's too small — there will be space between the orange and the cup, which could make it seem like the cup is too big. If you can't get a cup that's big enough for the orange to fit in, a flat plate (i.e. 40A) is the second-best option since at least it won't have weird gaps. This leads to women underestimating their true cup size.