Different bras have different cutting. Even same brand, different model also will have different cutting sometimes.
Measurements may not be as accurate cause everyone boobs and body size are different. Sometimes we go to the shop and get them to measure, but we still have to try on a few sizes to find the right bra size for the exact bra.

Unless the bra that is sold online is the same brand that we usually buy, then we most likely won't know the correct size.