If you are a 32" bust, it does not necessarily mean you are an A cup, no. But being an A cup means at least a 4 or 5" difference between your underbust and your bust. If you have a bigger cup size than this, the difference between your underbust measurement and the underbust measurement of the garment you knit (in relation to your bust size) will be even greater than 5". For example, if you are a B it will be 6" and if a C than 7" and so forth, so I was just saying that the smallest difference would be 4 to 5".
Think of it like this. Cup sizes are measurements of the difference between the width across your bust and the width across your underbust. If the difference is less than 5 or 6", you are an A cup, 6-7", a B cup, and so forth.
Now, if you look at the design's schematic and check the measurement of the underbust band, you will see that the underbust band measurement is the same as the bust measurement she tells you to knit for. That is, if you knit the size for the 32" bust, the underbust band will also be 32". Unless you actually have no breasts, one's underbust size and bust size is not the same size. Even if you have small breasts, like an A cup, there is generally at least 4-5", which is what I was saying, and if you have a bigger cup size, the difference between your bust and underbust will be even greater. This is what I meant when I said at least a 4-5" difference. For you at a D cup, it will be closer to 8". But for no person is it 0", and this is all I was saying.
And in fact you can see this in the example the model is wearing as the underbust band is in fact sagging.