The bra calculators are *at best* an educated guess, and each method will work better for some women (and body shapes) than others. The only real guarantee is to be professionally fitted.
Failing that, go and try on a bunch of bras and see if they help you triangulate. And don't forget about "sister sizes" : a 34C has the same cup volume as a 36B and 38A. (Basically, every time you go up/down a band a size, you will go down/up one cup size.) If you don't fill the cup, it's a sign that the bra either doesn't fit or the style doesn't work with your body, or both. It doesn't mean your breasts are defective!
I don't see any reason why our breasts won't fill out like a cis woman's, but it takes time, just as it does for them. We start out with those super perky teenage cone breasts, as others have said, and with time fat is layered on and they start to round out and be less "rock hard." I definitely have nicely rounded/normal-shaped breasts now. It is true that they're a bit widely spaced on my chest, but not out of the range of what I've seen on cis women either. We *tend* to have slightly broader chests than a cis woman of the same height/weight/body frame, but there's SO much variation that we'll usually fit easily within the total range.
