If their swimsuits run the same way their lingerie does, the cups sizes can be off.
I have to try on several of the same sizes for everything in the lingerie department. The cup sizes seem to run really small. Even the manager at our local store comments about the hit or miss of cups fitting. They had one outfit at Christmas time, that for every 1 they sold, they had to send 4 back, because of the cups running too small.
I wouldn't order a swinsuit on line. Go into the store and try it on. Now for shirts and sweaters, I have no issue ordering on line.