This is interesting indeed.
Maybe you forgot to add 4 inches to calculate your band size? Are you sure your underbust measures 32 inches?
I ask this because I'm 34 band (just 2 inches below you, so while noticeable our ribcages are not so different) and after more than 5 years on HRT I just got small B cups (they looked quite flat indeed). After getting BA I went from B to DD (bust measure went from 35 to 40) and they look anything but flat/modest. Have to say I got almost no muscle on my chest and I am (in my doctors words) "very petite"; my BMI is at 19.
That being said, if your measurings are correct and your chest looks flat even when correctly measuring for a D cup this can be due to a number of reasons:
- Ribcage being noticeable wider at bust height. A very shaped "V" torso may have a noticeable difference in ribcage width depending if you measure it at underbust height or at bust height.
- Well developed pectoralis muscles can give an obvious increase in volume at bust and also can mess with the shape of the breast thus making it look wider, flatter and more masculine even if there is an evident fat deposit on the breast.
- If fat is stored not just at bust height but also above the bust and in your back then breast may look flattish.
- Also being a little overweight can soften a lot the appearance of breasts.
So even if you measured it correctly, I dunno, but it may be feasible to get that cup size while your chest is actually looking kinda flat. Also, as another member said, the bigger the ribcage is, the less flashy the same cup size looks on it (they got fixed values).