THOSE are guidelines, they are not the perfect one size that will fit all.
Best answer is try them on @ the stores, even a size recommended (for me anyway), they are still too tight around the band, (add extensions found @ drug stores or fabric stores, problem solved), or cups too big or too small.
So, let's say you wear a size 32B, another brand or style won't be a 32B. Just saying.
I also suggest, if you're buying bras online, there are stores that ships FREE, and accepts return or exchange FREE shipping.
One person, told me, she can ONLY FIT in Frederick's of Hollywood!
I did a review on shopping for bras (in various store and online), on Yelp in my city.