[Pansy-Mae]

My questions above have been triggered by the fact that in the last week my band size seems to have reduced all of a sudden, even if my weight hasn't changed, and my "breasts" seem to have dropped a bit. Yesterday I tried on my 38C, bought because they were cheap and not because I think I'll ever fill them, and for the first time my nipples were touching the fabric of the cup and the band seemed looser than usual.

There is your answer!  
 I had a similar puzzle some years ago, and eventually it dawned on me that since my breasts had matured somewhat they dropped as well and my nipples were noticeably lower than they used to be. A male chest/ribcage  is a different shape to a female one, as you said before, but it isn't just broader, it also tapers down towards the waist. So as your breasts elongate and hang lower the undercrease circumference, ie the bra band measurement, also moves down and, depending on the actual amount of ribcage taper and the amount of fat around the torso, your bra basnd size will also  reduce slightly. This doesn't in itself alter the cup size, but that will probabaly change as well although not so noticeably.
This is a picture that I posted on my threa back in 2012 to show what I mean.