This is one of the commonest questions I come across! Implant sizing is very subjective. Here are links to two of my patient's galleries who were both C cups after breast augmentation.
The first patient had 225 cc implants and the second had 300 cc implants.
There is no standardization of bra cup sizes. A woman can be measured a C cup, only to go across the street to another company and be measured differently. Similarly, implants produce a certain size based on the amount of pre-existing breast tissue. Besides, there is the variation of body frame, height and weight. Therefore a given implant could very well be a C cup in one woman and a B or D cup in another.
At Solomon plastic surgery, we spend a lot of time discussing desired outcomes and matching an implant to a prospective patient based on detailed measurements taken at consultation.