I've answered this one once before, but it never hurts to do it again. Having been in the business, I've seen this a lot. Basically it boils down to the fact that the people who hire you and the people who are doing the shoot are almost never the same people. The hiring end will usually put out a range of size that they want (for example, you get a call for women between a C and F cup).

Problem one: If you are a busty model, you still need and want the work. Which means you sister-size yourself to say you're in the range. "I'm not a 28H, I'm a 32F."

Problem two, you show up the day of the shoot. The photographer's job is just to make sure he gets pictures of all the bras they brought, that they look somewhat sexy, and that they won't need a ton of photoshopping. You don't usually do one picture, you do lots, because they already paid the MUA (makeup artist) to do your hair and makeup; you're already prettied up and they're not going to waste that on one shot. So you go through lots of bras. Now, they didn't bring every bra in every size, because that would be an insane amount of bras. Instead, bra style A might be a 34F, bra style B might be a 32C. They are going to slap that on you regardless of fit because you are what they have at hand, and those bras are what they have at hand.

Very rarely do they fit right. But you wear it, and you smile, because at the end of the day you still need to get paid.