Style vs Size
The style of bra plays a significant role and it can be hard to reconcile style and size. Not all styles will suit you. If you're like me, you have more fullness in the bottom of the breast than in the top, causing some styles of bras to gap, even if the band and underwire fit great. I found some bras would technically "fit" but the way the cups and/or straps were styled just wasn't flattering. They'd create back bulges or the wires would come too far up under my arm or the cup pulled awkwardly. So I moved on.
Some women have wider breasts, fullness in the top, or narrower breasts. If the sizing seems good but the cups still aren't quite right, try a different style, and not just more sizes in the same style.
A woman's breasts will change many times over her lifetime. Puberty, pregnancy, breastfeeding, weight gain/loss, age. Bras must evolve with the boobs. And they have. Technology has created nearly weightless bras that do a wonderful job of supporting and shaping us. Who knows what the future will bring.
I hope you've learned a little from my bra quest thus far. I'll keep you posted as further events warrant!